/* Physics 9630 Hub — "night lab notebook"
   Dark only. Warm ink pages, faint graph paper, a textbook serif for headings,
   a typewriter mono for labels and readings. Each unit is coloured by a real
   spectral emission line:
     Unit 1  hydrogen-α 656 nm   Unit 2  sodium 589 nm
     Unit 3  mercury 546 nm      Unit 4  hydrogen-β 486 nm                    */

/* ---------- fonts (self-hosted, SIL OFL — see ../fonts/LICENSE.txt) ---------- */
@font-face { font-family: "Fraunces"; font-style: normal; font-weight: 100 900; font-display: swap;
  src: url(../fonts/fraunces-latin-full-normal.woff2) format("woff2-variations"), url(../fonts/fraunces-latin-full-normal.woff2) format("woff2");
  unicode-range: U+0000-00FF, U+0131, U+0152-0153, U+02BB-02BC, U+02C6, U+02DA, U+02DC, U+0304, U+0308, U+0329, U+2000-206F, U+20AC, U+2122, U+2191, U+2193, U+2212, U+2215, U+FEFF, U+FFFD; }
@font-face { font-family: "Fraunces"; font-style: italic; font-weight: 100 900; font-display: swap;
  src: url(../fonts/fraunces-latin-full-italic.woff2) format("woff2-variations"), url(../fonts/fraunces-latin-full-italic.woff2) format("woff2");
  unicode-range: U+0000-00FF, U+0131, U+0152-0153, U+02BB-02BC, U+02C6, U+02DA, U+02DC, U+0304, U+0308, U+0329, U+2000-206F, U+20AC, U+2122, U+2191, U+2193, U+2212, U+2215, U+FEFF, U+FFFD; }
@font-face { font-family: "Atkinson Hyperlegible"; font-style: normal; font-weight: 400; font-display: swap; src: url(../fonts/atkinson-hyperlegible-latin-400-normal.woff2) format("woff2"); }
@font-face { font-family: "Atkinson Hyperlegible"; font-style: italic; font-weight: 400; font-display: swap; src: url(../fonts/atkinson-hyperlegible-latin-400-italic.woff2) format("woff2"); }
@font-face { font-family: "Atkinson Hyperlegible"; font-style: normal; font-weight: 700; font-display: swap; src: url(../fonts/atkinson-hyperlegible-latin-700-normal.woff2) format("woff2"); }
@font-face { font-family: "Atkinson Hyperlegible"; font-style: italic; font-weight: 700; font-display: swap; src: url(../fonts/atkinson-hyperlegible-latin-700-italic.woff2) format("woff2"); }
@font-face { font-family: "IBM Plex Mono"; font-style: normal; font-weight: 400; font-display: swap; src: url(../fonts/ibm-plex-mono-latin-400-normal.woff2) format("woff2"); }
@font-face { font-family: "IBM Plex Mono"; font-style: normal; font-weight: 500; font-display: swap; src: url(../fonts/ibm-plex-mono-latin-500-normal.woff2) format("woff2"); }

/* ---------- tokens ---------- */
:root {
  color-scheme: dark;
  --ink: #12110f;          /* the page */
  --ink-deep: #0c0b0a;     /* wells: simulations, figures */
  --ink-raised: #191714;   /* panels */
  --ink-high: #23201c;     /* hover, inputs */
  --rule: rgba(236, 228, 214, .1);
  --rule-strong: rgba(236, 228, 214, .21);
  --paper: #ece4d6;        /* main text: chalk on slate */
  --paper-bright: #f8f1e4;
  --pencil: #a69d8d;       /* secondary text */
  --graphite: #776f63;     /* tertiary text */
  --u1: #ff8a73;           /* H-α 656 nm */
  --u2: #ffc15e;           /* Na 589 nm */
  --u3: #9ee67f;           /* Hg 546 nm */
  --u4: #6cc8f5;           /* H-β 486 nm */
  --lavender: #c9a7ff;
  --teal: #6fe0cf;
  --good: #9ee67f;
  --bad: #ff8a73;
  --accent: var(--u2);     /* sodium amber outside the units */
  --serif: "Fraunces", "Iowan Old Style", "Palatino Linotype", Palatino, Georgia, serif;
  --sans: "Atkinson Hyperlegible", "Segoe UI", system-ui, -apple-system, Roboto, sans-serif;
  --mono: "IBM Plex Mono", ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;
  --header-h: 66px;
  --graph: linear-gradient(rgba(236, 228, 214, .045) 1px, transparent 1px) 0 0 / 22px 22px,
           linear-gradient(90deg, rgba(236, 228, 214, .045) 1px, transparent 1px) 0 0 / 22px 22px;
  /* names the simulations read (sims-core.js) */
  --text: #ece4d6; --muted: #a69d8d; --surface: #191714; --surface-2: #23201c;
  --sim-bg: #0e0d0b; --sim-grid: rgba(236, 228, 214, .075); --sim-axis: #a69d8d;
}
body[data-unit="u1"] { --accent: var(--u1); }
body[data-unit="u2"] { --accent: var(--u2); }
body[data-unit="u3"] { --accent: var(--u3); }
body[data-unit="u4"] { --accent: var(--u4); }
.uc-1 { --uc: var(--u1); } .uc-2 { --uc: var(--u2); } .uc-3 { --uc: var(--u3); } .uc-4 { --uc: var(--u4); }

/* ---------- base ---------- */
*, *::before, *::after { box-sizing: border-box; }
html { background: var(--ink); -webkit-text-size-adjust: 100%; scroll-padding-top: calc(var(--header-h) + 64px); }
[hidden] { display: none !important; }
body {
  margin: 0; background: var(--ink); color: var(--paper); font-family: var(--sans);
  font-size: 17px; line-height: 1.7; -webkit-font-smoothing: antialiased; text-rendering: optimizeLegibility;
  position: relative;
}
/* paper grain behind everything */
body::before {
  content: ""; position: fixed; inset: 0; z-index: -1; pointer-events: none; opacity: .07;
  background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='180' height='180'%3E%3Cfilter id='n'%3E%3CfeTurbulence type='fractalNoise' baseFrequency='.85' numOctaves='2' stitchTiles='stitch'/%3E%3CfeColorMatrix values='0 0 0 0 1 0 0 0 0 .95 0 0 0 0 .88 0 0 0 .6 0'/%3E%3C/filter%3E%3Crect width='100%25' height='100%25' filter='url(%23n)'/%3E%3C/svg%3E");
}
::selection { background: var(--accent); color: var(--ink); }
img, svg, canvas, video, iframe { max-width: 100%; }
a { color: inherit; text-decoration: underline; text-decoration-color: var(--accent); text-decoration-thickness: 1.5px; text-underline-offset: .22em; }
a:hover { color: var(--accent); }
h1, h2, h3, h4 { font-family: var(--serif); font-weight: 600; line-height: 1.14; letter-spacing: -.012em; margin: 1.6em 0 .5em; color: var(--paper-bright);
  font-variation-settings: "opsz" 96, "SOFT" 40, "WONK" 0; }
h1 { font-size: clamp(2rem, 1.35rem + 2.6vw, 3.2rem); letter-spacing: -.022em; margin-top: .3em; font-variation-settings: "opsz" 144, "SOFT" 60, "WONK" 1; }
h2 { font-size: 1.7rem; }
h3 { font-size: 1.24rem; font-variation-settings: "opsz" 36, "SOFT" 40; }
h4 { font-size: 1.06rem; }
h1 em, h2 em { font-style: italic; font-weight: 500; color: var(--accent); }
p { margin: .6em 0 .95em; }
ul, ol { padding-left: 1.35em; }
li { margin: .3em 0; }
hr { border: 0; border-top: 1px solid var(--rule); margin: 2.4em 0; }
strong { font-weight: 700; color: var(--paper-bright); }
code { font-family: var(--mono); font-size: .86em; background: var(--ink-high); padding: .08em .35em; }
kbd { font-family: var(--mono); font-size: .82em; border: 1px solid var(--rule-strong); border-bottom-width: 2px; padding: 0 .35em; }
.sr-only { position: absolute !important; width: 1px; height: 1px; overflow: hidden; clip: rect(0 0 0 0); white-space: nowrap; }
.skip { position: absolute; left: -999px; top: 8px; z-index: 100; background: var(--accent); color: var(--ink); padding: 8px 12px; font-family: var(--mono); font-size: .8rem; }
.skip:focus { left: 12px; }
:focus-visible { outline: 1.5px dashed var(--accent); outline-offset: 3px; }
.label, .label-head, .kicker, .eyebrow {
  font-family: var(--mono); font-weight: 500; font-size: .72rem; letter-spacing: .16em; text-transform: uppercase; color: var(--pencil);
  font-variation-settings: normal;
}
.label-head { display: block; margin: 0 0 14px; padding-bottom: 8px; border-bottom: 1px solid var(--rule-strong); color: var(--accent); line-height: 1.4; }
.kicker { color: var(--accent); margin: 0; }
.eyebrow { display: inline-flex; align-items: center; gap: 12px; margin: 0; }
.eyebrow::before { content: ""; width: 28px; height: 1px; background: currentColor; }
.fine { color: var(--pencil); font-size: .92rem; max-width: 75ch; }

/* ---------- form controls ---------- */
input[type="checkbox"] {
  -webkit-appearance: none; appearance: none; width: 15px; height: 15px; margin: 0; flex: none; cursor: pointer;
  border: 1.5px solid var(--pencil); background: transparent; display: inline-grid; place-content: center; vertical-align: -2px;
}
input[type="checkbox"]::after { content: ""; width: 8px; height: 4px; border-left: 2px solid var(--ink); border-bottom: 2px solid var(--ink); transform: translate(.5px, -1px) rotate(-45deg); opacity: 0; }
input[type="checkbox"]:checked { background: var(--accent); border-color: var(--accent); }
input[type="checkbox"]:checked::after { opacity: 1; }
select {
  -webkit-appearance: none; appearance: none; border-radius: 0; cursor: pointer;
  background: var(--ink-high) url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='10' height='6'%3E%3Cpath d='M0 0l5 6 5-6z' fill='%23a69d8d'/%3E%3C/svg%3E") right 9px center no-repeat;
  border: 1px solid var(--rule-strong); color: var(--paper); font-family: var(--mono); font-size: .78rem; padding: 6px 28px 6px 9px;
}
select:hover { border-color: var(--pencil); }
input[type="range"] { -webkit-appearance: none; appearance: none; width: 100%; height: 20px; background: transparent; cursor: pointer; margin: 0; }
input[type="range"]::-webkit-slider-runnable-track { height: 2px; background: var(--rule-strong); }
input[type="range"]::-webkit-slider-thumb { -webkit-appearance: none; width: 11px; height: 11px; margin-top: -4.5px; background: var(--accent); border: 0; transform: rotate(45deg); box-shadow: 0 0 0 3px var(--ink-raised), 0 0 10px var(--accent); }
input[type="range"]::-moz-range-track { height: 2px; background: var(--rule-strong); }
input[type="range"]::-moz-range-thumb { width: 10px; height: 10px; border: 0; border-radius: 0; background: var(--accent); transform: rotate(45deg); box-shadow: 0 0 0 3px var(--ink-raised), 0 0 10px var(--accent); }
.btn {
  display: inline-flex; align-items: center; gap: 8px; padding: 9px 14px; border: 1px solid var(--rule-strong); border-radius: 0;
  background: transparent; color: var(--paper); font-family: var(--mono); font-size: .72rem; font-weight: 500; letter-spacing: .1em; text-transform: uppercase;
  text-decoration: none; cursor: pointer; line-height: 1.2;
}
.btn:hover { border-color: var(--accent); color: var(--accent); }
.btn-primary { background: var(--accent); border-color: var(--accent); color: var(--ink); }
.btn-primary:hover { color: var(--ink); filter: brightness(1.08); }
.btn-ghost { background: transparent; }
.btn-sm { padding: 7px 11px; }
.icon-btn { display: inline-grid; place-items: center; width: 40px; height: 40px; border: 1px solid var(--rule-strong); background: var(--ink); color: var(--paper); cursor: pointer; }
.icon-btn:hover { border-color: var(--accent); color: var(--accent); }
.icon-btn svg { width: 18px; height: 18px; }
.check { display: inline-flex; align-items: center; gap: 9px; cursor: pointer; }

/* ---------- layout ---------- */
.wrap { width: 100%; max-width: 1180px; margin: 0 auto; padding: 0 20px; }
.wrap.narrow { max-width: 900px; }
main { min-height: 60vh; }

/* ---------- header ---------- */
.site-header { position: sticky; top: 0; z-index: 40; background: var(--ink); border-bottom: 1px solid var(--rule-strong); }
.spectrum-strip { display: flex; height: 3px; }
.spectrum-strip i { flex: 1; }
.spectrum-strip i:nth-child(1) { background: var(--u4); }
.spectrum-strip i:nth-child(2) { background: var(--u3); }
.spectrum-strip i:nth-child(3) { background: var(--u2); }
.spectrum-strip i:nth-child(4) { background: var(--u1); }
.site-header .wrap { display: flex; align-items: center; gap: 24px; height: calc(var(--header-h) - 4px); }
.brand { display: flex; align-items: center; gap: 13px; color: var(--paper); text-decoration: none; white-space: nowrap; }
.brand:hover { color: var(--paper); }
.tile {
  position: relative; display: inline-grid; place-items: center; flex: none; width: 40px; height: 40px;
  border: 1.5px solid var(--paper); font-family: var(--serif); font-weight: 650; font-size: 19px; line-height: 1; padding-top: 5px;
  color: var(--paper-bright); box-shadow: inset 0 -3px 0 var(--accent);
}
.tile small { position: absolute; top: 3px; left: 4px; font-family: var(--mono); font-size: 7.5px; letter-spacing: .02em; color: var(--pencil); font-weight: 500; }
.brand:hover .tile { border-color: var(--accent); }
.wordmark { display: flex; flex-direction: column; min-width: 0; font-family: var(--serif); font-weight: 600; font-size: 1.12rem; letter-spacing: -.01em; line-height: 1.15; font-variation-settings: "opsz" 48, "SOFT" 50; }
.wordmark i { font-style: italic; font-weight: 500; color: var(--accent); }
.wordmark small { font-family: var(--mono); font-weight: 400; font-size: .6rem; letter-spacing: .14em; text-transform: uppercase; color: var(--pencil); margin-top: 3px; }
.main-nav { display: flex; gap: 2px; margin-left: auto; overflow-x: auto; scrollbar-width: none; }
.main-nav::-webkit-scrollbar { display: none; }
.main-nav a {
  display: inline-flex; align-items: center; gap: 8px; padding: 9px 11px 8px; white-space: nowrap;
  font-family: var(--mono); font-size: .74rem; font-weight: 500; letter-spacing: .1em; text-transform: uppercase;
  color: var(--pencil); text-decoration: none; border-bottom: 2px solid transparent;
}
.main-nav a i { width: 2px; height: 13px; background: var(--c); box-shadow: 0 0 6px var(--c); }
.main-nav a[data-u="1"] { --c: var(--u1); }
.main-nav a[data-u="2"] { --c: var(--u2); }
.main-nav a[data-u="3"] { --c: var(--u3); }
.main-nav a[data-u="4"] { --c: var(--u4); }
.main-nav a:hover { color: var(--paper); }
.main-nav a[aria-current="page"] { color: var(--paper-bright); border-bottom-color: var(--c, var(--accent)); }

/* ---------- breadcrumbs ---------- */
.crumbs { display: flex; flex-wrap: wrap; align-items: center; gap: 4px 10px; margin: 22px 0 0; font-family: var(--mono); font-size: .7rem; letter-spacing: .1em; text-transform: uppercase; color: var(--graphite); }
.crumbs a { color: var(--pencil); text-decoration: none; }
.crumbs a:hover { color: var(--accent); }
.crumbs .sep { color: var(--graphite); }

/* ---------- home: hero ---------- */
.hero { display: grid; grid-template-columns: minmax(0, 1.35fr) minmax(0, 1fr); gap: 56px; align-items: center; padding: 70px 0 40px; }
.hero h1 { font-size: clamp(2.7rem, 1.6rem + 4.3vw, 4.9rem); line-height: .98; margin: .32em 0 .38em; max-width: 12ch; }
.lead { font-size: 1.12rem; color: var(--pencil); max-width: 58ch; }
.search { position: relative; max-width: 540px; margin-top: 30px; }
.search svg { position: absolute; left: 0; top: 50%; width: 20px; height: 20px; transform: translateY(-50%); color: var(--accent); }
.search input {
  width: 100%; padding: 12px 6px 12px 34px; border: 0; border-bottom: 1.5px solid var(--rule-strong); border-radius: 0; background: transparent;
  color: var(--paper-bright); font-family: var(--serif); font-style: italic; font-size: 1.3rem; font-variation-settings: "opsz" 48;
}
.search input::placeholder { color: var(--graphite); }
.search input:focus { outline: none; border-bottom-color: var(--accent); }
.search input::-webkit-search-cancel-button { filter: invert(.7); }
.search-results { position: absolute; left: 0; right: 0; top: calc(100% + 8px); z-index: 20; display: none; max-height: 360px; overflow: auto; background: var(--ink-raised); border: 1px solid var(--rule-strong); box-shadow: 0 18px 40px rgba(0, 0, 0, .55); }
.search-results.open { display: block; }
.search-results a { display: block; padding: 11px 14px; color: var(--paper); text-decoration: none; border-bottom: 1px solid var(--rule); }
.search-results a:last-child { border-bottom: 0; }
.search-results a:hover, .search-results a.sel { background: var(--ink-high); color: var(--paper-bright); }
.search-results small { display: block; font-family: var(--mono); font-size: .68rem; letter-spacing: .08em; text-transform: uppercase; color: var(--pencil); }

/* the spectroscope */
.spectro { margin: 0; padding: 18px 20px 14px; border: 1px solid var(--rule-strong); background: var(--graph), var(--ink-deep); position: relative; }
.spectro::before, .spectro::after { content: ""; position: absolute; width: 10px; height: 10px; border-color: var(--pencil); border-style: solid; }
.spectro::before { top: -1px; left: -1px; border-width: 1.5px 0 0 1.5px; }
.spectro::after { bottom: -1px; right: -1px; border-width: 0 1.5px 1.5px 0; }
.spectro .label { display: block; margin-bottom: 14px; }
.spectro-body { position: relative; height: 380px; }
.spectro .band {
  position: absolute; left: 0; top: 0; bottom: 0; width: 54px; border: 1px solid var(--rule-strong);
  background: linear-gradient(180deg, rgba(130, 70, 255, .16), rgba(40, 110, 255, .14) 22%, rgba(30, 200, 210, .12) 38%, rgba(70, 220, 90, .12) 52%,
              rgba(240, 220, 60, .12) 64%, rgba(255, 140, 40, .12) 78%, rgba(220, 40, 30, .14));
}
.sl { position: absolute; left: 0; right: 0; top: var(--y); transform: translateY(-50%); display: grid; grid-template-columns: 54px minmax(0, 1fr); column-gap: 18px; align-items: center; text-decoration: none; color: var(--paper); }
.sl::before { content: ""; grid-row: 1 / span 2; width: 54px; height: 3px; background: var(--uc); box-shadow: 0 0 8px var(--uc), 0 0 22px var(--uc); transition: box-shadow .2s, height .2s; }
.sl::after { content: ""; position: absolute; left: 54px; width: 14px; top: 50%; border-top: 1px dotted var(--uc); opacity: .7; }
.sl span { display: block; grid-column: 2; }
.sl .nm { font-family: var(--mono); font-size: .68rem; letter-spacing: .06em; color: var(--uc); line-height: 1.5; }
.sl .un { font-family: var(--serif); font-size: 1.04rem; line-height: 1.25; font-variation-settings: "opsz" 24; }
.sl .un b { font-weight: 650; margin-right: 4px; }
.sl:hover { color: var(--paper-bright); }
.sl:hover::before { height: 5px; box-shadow: 0 0 10px var(--uc), 0 0 34px var(--uc), 0 0 60px var(--uc); }
.sl:hover .un { color: var(--uc); }
.spectro-note { margin: 12px 0 0; font-size: .8rem; color: var(--graphite); line-height: 1.5; }

.readout-line { display: flex; flex-wrap: wrap; gap: 6px 26px; padding: 14px 0; margin: 10px 0 0; border-top: 1px solid var(--rule-strong); border-bottom: 1px solid var(--rule-strong);
  font-family: var(--mono); font-size: .74rem; letter-spacing: .08em; text-transform: uppercase; color: var(--pencil); }
.readout-line b { font-family: var(--serif); font-size: 1.35rem; font-weight: 600; letter-spacing: 0; color: var(--paper-bright); margin-right: 6px; text-transform: none; font-variant-numeric: lining-nums; }

/* ---------- section heads ---------- */
.section-head { display: grid; grid-template-columns: 150px minmax(0, 1fr); gap: 6px 30px; align-items: baseline; margin: 74px 0 26px; }
.section-head .label { color: var(--accent); }
.section-head h2 { margin: 0; font-size: clamp(1.7rem, 1.25rem + 1.6vw, 2.5rem); font-variation-settings: "opsz" 144, "SOFT" 60; }
.section-head p { grid-column: 2; margin: 2px 0 0; color: var(--pencil); }

/* ---------- home: unit index ---------- */
.unit-index { list-style: none; margin: 0; padding: 0; border-top: 1px solid var(--rule-strong); }
.unit-index li { margin: 0; border-bottom: 1px solid var(--rule-strong); }
.unit-index a { position: relative; display: grid; grid-template-columns: 150px minmax(0, 1fr) 230px; gap: 10px 30px; padding: 28px 0; color: inherit; text-decoration: none; }
.unit-index .no {
  font-family: var(--serif); font-weight: 700; font-size: 5rem; line-height: .82; color: transparent; -webkit-text-stroke: 1.2px var(--uc);
  font-variation-settings: "opsz" 144, "SOFT" 100, "WONK" 1; transition: color .2s;
}
.unit-index a:hover .no { color: var(--uc); text-shadow: 0 0 30px color-mix(in srgb, var(--uc) 40%, transparent); }
.unit-index .t { display: block; font-family: var(--serif); font-size: 1.7rem; font-weight: 600; line-height: 1.15; color: var(--paper-bright); font-variation-settings: "opsz" 72, "SOFT" 50; }
.unit-index a:hover .t { color: var(--uc); }
.unit-index .d { display: block; margin-top: 6px; color: var(--pencil); font-size: .98rem; max-width: 60ch; }
.unit-index .meta { display: flex; flex-direction: column; gap: 5px; padding-top: 6px; font-family: var(--mono); font-size: .7rem; letter-spacing: .08em; text-transform: uppercase; color: var(--pencil); }
.unit-index .meta span:first-child { color: var(--uc); }
.unit-index .meta b { color: var(--paper-bright); font-weight: 500; }
.progress-wrap { display: flex; align-items: center; gap: 12px; margin-top: 14px; max-width: 420px; }
.progress-wrap .pp-count { white-space: nowrap; }
.progress { display: block; flex: 1; height: 2px; background: var(--rule-strong); margin: 0; }
.progress > i { display: block; height: 100%; width: 0; background: var(--uc, var(--accent)); box-shadow: 0 0 8px var(--uc, var(--accent)); transition: width .5s ease; }
.pp-count { font-family: var(--mono); font-size: .7rem; letter-spacing: .06em; color: var(--pencil); }

/* ---------- home: inside a chapter ---------- */
.inside { list-style: none; margin: 0; padding: 0; counter-reset: ins; display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); gap: 0 44px; }
.inside li { counter-increment: ins; position: relative; margin: 0; padding: 18px 0 18px 54px; border-top: 1px solid var(--rule); }
.inside li::before { content: counter(ins, lower-roman) "."; position: absolute; left: 0; top: 16px; font-family: var(--serif); font-style: italic; font-size: 1.35rem; color: var(--accent); }
.inside h3 { margin: 0 0 3px; font-size: 1.15rem; }
.inside p { margin: 0; color: var(--pencil); font-size: .96rem; }

/* ---------- tables ---------- */
table.data { width: 100%; border-collapse: collapse; font-size: .95rem; }
table.data th, table.data td { text-align: left; vertical-align: top; padding: 10px 14px 10px 0; border-bottom: 1px solid var(--rule); }
table.data thead th { font-family: var(--mono); font-weight: 500; font-size: .7rem; letter-spacing: .12em; text-transform: uppercase; color: var(--pencil); border-bottom: 1px solid var(--rule-strong); }
table.data tbody th { font-family: var(--mono); font-weight: 500; font-size: .72rem; letter-spacing: .08em; text-transform: uppercase; color: var(--pencil); width: 42%; padding-top: 13px; }
table.data a { text-decoration-color: var(--accent); }
.table-scroll { overflow-x: auto; -webkit-overflow-scrolling: touch; margin: 1em 0; position: relative; }
.katex { position: relative; }

/* ---------- unit page ---------- */
.unit-head { display: grid; grid-template-columns: auto minmax(0, 1fr); gap: 10px 40px; align-items: start; padding: 26px 0 8px; }
.big-no, .ch-no {
  font-family: var(--serif); font-weight: 700; line-height: .8; color: transparent; -webkit-text-stroke: 1.4px var(--accent);
  font-variation-settings: "opsz" 144, "SOFT" 100, "WONK" 1; letter-spacing: -.03em; user-select: none;
}
.big-no { font-size: clamp(5.5rem, 3.5rem + 7vw, 10rem); margin-top: .08em; }
.unit-head h1 { margin: .2em 0 .25em; }
.unit-head .readout-line { margin-top: 18px; }
.unit-progress { display: flex; align-items: center; gap: 14px; max-width: 460px; margin-top: 16px; }
.two-col { display: grid; grid-template-columns: minmax(0, 1fr) 320px; gap: 56px; align-items: start; margin-top: 44px; }
.chapter-list { list-style: none; margin: 0; padding: 0; counter-reset: none; }
.chapter-list li { margin: 0; }
.chapter-list a { display: flex; align-items: baseline; gap: 16px; padding: 14px 2px 13px; border-bottom: 1px solid var(--rule); color: var(--paper); text-decoration: none; }
.chapter-list .n { flex: none; width: 3.3em; font-family: var(--mono); font-size: .8rem; color: var(--accent); }
.chapter-list .t { font-family: var(--serif); font-size: 1.14rem; font-weight: 560; line-height: 1.3; color: var(--paper-bright); font-variation-settings: "opsz" 30, "SOFT" 40; }
.chapter-list .t small { display: block; margin-top: 2px; font-family: var(--mono); font-size: .66rem; font-weight: 400; letter-spacing: .1em; text-transform: uppercase; color: var(--graphite); }
.chapter-list .dots { flex: 1; min-width: 24px; border-bottom: 1.5px dotted var(--rule-strong); transform: translateY(-.35em); }
.chapter-list .m { flex: none; font-family: var(--mono); font-size: .7rem; letter-spacing: .05em; color: var(--pencil); white-space: nowrap; }
.chapter-list .done { flex: none; font-family: var(--mono); font-size: .68rem; letter-spacing: .08em; text-transform: uppercase; color: var(--good); }
.chapter-list a:hover .t { color: var(--accent); }
.chapter-list a:hover .dots { border-bottom-color: var(--accent); }
.sheet .label-head + table.data { margin-bottom: 30px; }
.papers-used { color: var(--pencil); font-size: .92rem; margin: 0; }

/* ---------- chapter page ---------- */
.chapter-head { display: grid; grid-template-columns: auto minmax(0, 1fr); gap: 10px 34px; align-items: end; padding: 26px 0 4px; }
.ch-no { font-size: clamp(4.2rem, 2.8rem + 5vw, 7.6rem); }
.chapter-head h1 { margin: .18em 0 .35em; font-size: clamp(1.9rem, 1.3rem + 2.3vw, 3.1rem); line-height: 1.04; }
.chapter-head .sub { display: flex; flex-wrap: wrap; align-items: center; gap: 6px 22px; font-family: var(--mono); font-size: .72rem; letter-spacing: .08em; text-transform: uppercase; color: var(--pencil); }
.revised-toggle { display: inline-flex; align-items: center; gap: 9px; cursor: pointer; user-select: none; color: var(--paper); }
.revised-toggle input:checked { background: var(--good); border-color: var(--good); }

.tabbar { position: sticky; top: var(--header-h); z-index: 30; background: var(--ink); border-bottom: 1px solid var(--rule-strong); margin-top: 22px; }
.tabs { display: flex; overflow-x: auto; scrollbar-width: none; }
.tabs::-webkit-scrollbar { display: none; }
.tab {
  position: relative; flex: none; display: inline-flex; align-items: baseline; gap: 6px; padding: 17px 16px 15px; white-space: nowrap; cursor: pointer;
  background: transparent; border: 0; color: var(--pencil); font-family: var(--mono); font-size: .74rem; font-weight: 500; letter-spacing: .1em; text-transform: uppercase;
}
.tab svg { display: none; }
.tab .count { font-size: .64rem; color: var(--graphite); position: relative; top: -.45em; letter-spacing: .02em; }
.tab:hover { color: var(--paper); }
.tab[aria-selected="true"] { color: var(--paper-bright); }
.tab[aria-selected="true"]::after { content: ""; position: absolute; left: 14px; right: 14px; bottom: -1px; height: 3px; background: var(--accent); box-shadow: 0 0 12px var(--accent); }
.tab[aria-selected="true"] .count { color: var(--accent); }
.panel { padding: 30px 0 46px; }
.panel-intro { color: var(--pencil); max-width: 72ch; margin-top: 0; }

.learn-box { margin: 0 0 10px; padding: 18px 22px 14px; background: var(--graph), var(--ink-raised); border: 1px solid var(--rule); }
.learn-box h2 { margin: 0 0 10px; font-family: var(--mono); font-size: .72rem; font-weight: 500; letter-spacing: .16em; text-transform: uppercase; color: var(--accent); }
.learn-box ul { list-style: none; margin: 0; padding: 0; columns: 2 300px; column-gap: 34px; }
.learn-box li { position: relative; padding-left: 26px; margin: .2em 0 .45em; break-inside: avoid; font-size: .96rem; }
.learn-box li::before { content: ""; position: absolute; left: 0; top: .42em; width: 11px; height: 11px; border: 1.5px solid var(--accent); }

.notes-layout { display: grid; grid-template-columns: minmax(0, 1fr) 220px; gap: 48px; align-items: start; }
.toc { position: sticky; top: calc(var(--header-h) + 78px); max-height: calc(100vh - 170px); overflow: auto; font-size: .9rem; }
.toc h2 { margin: 0 0 10px; padding-bottom: 8px; border-bottom: 1px solid var(--rule-strong); font-family: var(--mono); font-size: .7rem; font-weight: 500; letter-spacing: .16em; text-transform: uppercase; color: var(--pencil); }
.toc ul { list-style: none; margin: 0; padding: 0; counter-reset: toc; }
.toc li { counter-increment: toc; margin: 0; }
.toc a { display: grid; grid-template-columns: 2.4em minmax(0, 1fr); padding: 5px 0; color: var(--pencil); text-decoration: none; line-height: 1.4; }
.toc a::before { content: "§" counter(toc); font-family: var(--mono); font-size: .7rem; color: var(--graphite); padding-top: .15em; }
.toc a:hover { color: var(--paper); }
.toc a.active { color: var(--paper-bright); }
.toc a.active::before { color: var(--accent); }

/* ---------- notes ---------- */
.prose { max-width: 70ch; counter-reset: sec fig; }
.prose > h2 { counter-increment: sec; margin: 2.3em 0 .55em; padding-top: 1.1em; border-top: 1px solid var(--rule); font-size: 1.95rem; font-variation-settings: "opsz" 144, "SOFT" 50; }
.prose > h2::before { content: "§ " counter(sec); display: block; margin-bottom: .55em; font-family: var(--mono); font-size: .7rem; font-weight: 500; letter-spacing: .16em; color: var(--accent); font-variation-settings: normal; }
.prose > h2:first-child { margin-top: 1.4em; }
.prose h3 { margin-top: 1.9em; font-size: 1.32rem; }
.prose ul li::marker { color: var(--accent); }
.prose ol li::marker { font-family: var(--mono); font-size: .82em; color: var(--accent); }
.prose .table-scroll { border-top: 1px solid var(--rule-strong); border-bottom: 1px solid var(--rule-strong); margin: 1.4em 0; }
.prose table { width: 100%; border-collapse: collapse; margin: 0; font-size: .95rem; }
.prose table th { text-align: left; vertical-align: bottom; padding: 11px 16px 8px 0; border-bottom: 1px solid var(--rule-strong); font-family: var(--mono); font-weight: 500; font-size: .7rem; letter-spacing: .1em; text-transform: uppercase; color: var(--pencil); }
.prose table td { text-align: left; vertical-align: top; padding: 9px 16px 9px 0; border-bottom: 1px solid var(--rule); }
.prose table tr:last-child td { border-bottom: 0; }
.prose figure { counter-increment: fig; margin: 1.9em 0; padding: 20px 20px 14px; background: var(--graph), var(--ink-deep); border: 1px solid var(--rule); }
.prose figure svg { display: block; width: 100%; height: auto; max-width: 560px; margin: 0 auto; color: var(--paper); }
.prose figcaption { margin-top: 12px; font-size: .88rem; color: var(--pencil); line-height: 1.55; }
.prose figcaption::before { content: "Fig. " counter(fig); margin-right: 10px; font-family: var(--mono); font-size: .68rem; letter-spacing: .14em; text-transform: uppercase; color: var(--accent); }
.prose .katex-display { overflow-x: auto; overflow-y: hidden; padding: 6px 0; margin: 1.1em 0; position: relative; }
.prose .katex { font-size: 1.08em; }
.prose figure text { font-family: var(--sans); }

/* the diagrams in the notes were drawn with a light palette: re-ink them */
.prose svg [fill="#2563eb"], .prose svg [fill="#0284c7"], .prose svg [fill="#0ea5e9"], .prose svg [fill="#38bdf8"] { fill: var(--u4); }
.prose svg [stroke="#2563eb"], .prose svg [stroke="#0ea5e9"] { stroke: var(--u4); }
.prose svg [fill="#dc2626"] { fill: var(--u1); }
.prose svg [stroke="#dc2626"] { stroke: var(--u1); }
.prose svg [fill="#16a34a"] { fill: var(--u3); }
.prose svg [stroke="#16a34a"] { stroke: var(--u3); }
.prose svg [fill="#ea580c"] { fill: #ffad66; }
.prose svg [stroke="#ea580c"] { stroke: #ffad66; }
.prose svg [fill="#7c3aed"] { fill: var(--lavender); }
.prose svg [stroke="#7c3aed"] { stroke: var(--lavender); }
.prose svg [fill="#94a3b8"], .prose svg [fill="#334155"] { fill: #8f877a; }
.prose svg [fill="#fff"] { fill: var(--ink); }
.prose svg [fill="#e5e7eb"] { fill: #cfc6b6; }
.prose svg [fill="#bae6fd"] { fill: var(--u4); }

/* callout boxes (python-markdown admonitions) */
.admonition { position: relative; margin: 1.7em 0; padding: 15px 20px 9px; background: var(--ink-raised); border: 1px solid var(--rule); }
.admonition > .admonition-title { margin: 0 0 6px; font-family: var(--mono); font-size: .72rem; font-weight: 500; letter-spacing: .14em; text-transform: uppercase; color: var(--c, var(--accent)); line-height: 1.5; }
.admonition > p:last-child, .admonition > ul:last-child, .admonition > ol:last-child { margin-bottom: .7em; }
.admonition.key > .admonition-title, .admonition.definition > .admonition-title {
  font-family: var(--serif); font-style: italic; font-size: 1.2rem; font-weight: 500; letter-spacing: 0; text-transform: none; color: var(--paper-bright); font-variation-settings: "opsz" 48;
}
.admonition.key > .admonition-title::before, .admonition.definition > .admonition-title::before {
  display: block; margin-bottom: 2px; font-family: var(--mono); font-style: normal; font-size: .68rem; letter-spacing: .16em; text-transform: uppercase; color: var(--c);
}
.admonition.key { --c: var(--accent); background: var(--graph), var(--ink-raised); border-color: color-mix(in srgb, var(--accent) 42%, transparent); }
.admonition.key > .admonition-title::before { content: "Key idea"; }
.admonition.definition { --c: var(--lavender); background: transparent; border: 0; border-left: 4px double var(--lavender); padding: 4px 0 4px 22px; }
.admonition.definition > .admonition-title::before { content: "Definition"; }
.admonition.example { --c: var(--paper); background: transparent; border: 0; border-left: 1px solid var(--rule-strong); padding: 2px 0 2px 24px; }
.admonition.example > .admonition-title::before { content: "✎  "; color: var(--accent); }
.admonition.tip { --c: var(--u2); border: 0; border-left: 2px solid var(--u2); background: color-mix(in srgb, var(--u2) 6%, var(--ink-raised)); }
.admonition.warning { --c: var(--u1); background: transparent; border: 1px dashed color-mix(in srgb, var(--u1) 55%, transparent); }
.admonition.warning > .admonition-title::before { content: "✕  "; }
.admonition.practical { --c: var(--teal); background: transparent; border-color: color-mix(in srgb, var(--teal) 38%, transparent); padding-top: 22px; }
.admonition.practical::before { content: ""; position: absolute; left: 0; right: 0; top: 0; height: 6px; background: repeating-linear-gradient(-45deg, var(--teal) 0 1.5px, transparent 1.5px 7px); opacity: .6; }
.admonition.summary { --c: var(--accent); background: transparent; border: 0; border-top: 2px solid var(--accent); border-bottom: 2px solid var(--accent); padding: 14px 2px 6px; }
.admonition.info, .admonition.note { --c: var(--pencil); }

/* ---------- interactive simulations ---------- */
.sim { margin: 0 0 36px; background: var(--ink-raised); border: 1px solid var(--rule-strong); }
.sim-head { display: flex; flex-wrap: wrap; align-items: baseline; justify-content: space-between; gap: 6px 14px; padding: 13px 18px 11px; border-bottom: 1px solid var(--rule); }
.sim-head h3 { margin: 0; font-size: 1.22rem; }
.sim-head .hint { font-family: var(--mono); font-size: .68rem; letter-spacing: .1em; text-transform: uppercase; color: var(--pencil); }
.sim-body { display: grid; grid-template-columns: minmax(0, 1fr) 300px; }
.sim-stage { position: relative; min-height: 280px; background: var(--sim-bg); }
.sim-stage canvas { display: block; width: 100%; height: 100%; touch-action: pan-y; }
.sim-stage canvas.drag { touch-action: none; cursor: grab; }
.sim-side { display: flex; flex-direction: column; gap: 14px; padding: 15px 16px 16px; border-left: 1px solid var(--rule); }
.ctrl label { display: flex; justify-content: space-between; gap: 10px; margin-bottom: 3px; font-size: .86rem; line-height: 1.35; color: var(--paper); }
.ctrl label output { flex: none; font-family: var(--mono); font-size: .76rem; color: var(--accent); }
.ctrl select { width: 100%; }
.ctrl-row { display: flex; flex-wrap: wrap; gap: 8px; }
.seg { display: flex; flex-wrap: wrap; border: 1px solid var(--rule-strong); }
.seg button {
  flex: 1 1 auto; padding: 7px 9px; border: 0; border-right: 1px solid var(--rule); background: transparent; cursor: pointer;
  color: var(--pencil); font-family: var(--mono); font-size: .7rem; letter-spacing: .04em; line-height: 1.35;
}
.seg button:last-child { border-right: 0; }
.seg button:hover { color: var(--paper); background: var(--ink-high); }
.seg button[aria-pressed="true"] { background: var(--accent); color: var(--ink); }
.readout { padding: 10px 12px; background: var(--ink-deep); border: 1px solid var(--rule); font-family: var(--mono); font-size: .74rem; line-height: 1.75; }
.readout .row { display: flex; align-items: baseline; gap: 8px; }
.readout .row span { flex: 1; display: flex; min-width: 0; color: var(--pencil); }
.readout .row span::after { content: ""; flex: 1; min-width: 12px; margin: 0 0 .32em 8px; border-bottom: 1px dotted var(--rule-strong); }
.readout .row b { flex: none; max-width: 58%; text-align: right; font-weight: 500; color: var(--paper-bright); }
.readout .note { margin-top: 7px; padding-top: 7px; border-top: 1px solid var(--rule); font-family: var(--sans); font-size: .84rem; line-height: 1.5; color: var(--pencil); }
.sim-caption { padding: 12px 18px 15px; border-top: 1px solid var(--rule); color: var(--pencil); font-size: .93rem; }
.sim-caption strong { color: var(--paper-bright); }
.sim .check { font-size: .86rem; line-height: 1.35; color: var(--paper); }
.ctrl label.check { justify-content: flex-start; margin-bottom: 0; }

/* ---------- video ---------- */
.video-wrap { position: relative; aspect-ratio: 16 / 9; max-width: 920px; background: #000; border: 1px solid var(--rule-strong); }
.video-wrap iframe { position: absolute; inset: 0; width: 100%; height: 100%; border: 0; }
.video-facade { position: absolute; inset: 0; width: 100%; display: grid; place-items: center; border: 0; cursor: pointer; color: var(--paper); background: #0c0b0a center / cover no-repeat; }
.video-facade::after { content: ""; position: absolute; inset: 0; background: linear-gradient(180deg, rgba(12, 11, 10, .15), rgba(12, 11, 10, .82)); }
.video-facade .play { position: relative; z-index: 1; display: grid; place-items: center; width: 78px; height: 78px; border-radius: 50%; border: 1.5px solid var(--paper); background: rgba(12, 11, 10, .45); transition: transform .2s, border-color .2s; }
.video-facade .play svg { width: 26px; height: 26px; margin-left: 4px; fill: var(--paper); }
.video-facade:hover .play { transform: scale(1.06); border-color: var(--accent); }
.video-facade:hover .play svg { fill: var(--accent); }
.video-facade .vt { position: absolute; z-index: 1; left: 20px; right: 20px; bottom: 16px; text-align: left; font-family: var(--serif); font-style: italic; font-size: 1.3rem; line-height: 1.25; color: var(--paper-bright); font-variation-settings: "opsz" 48; }
.video-meta { display: flex; flex-wrap: wrap; align-items: center; gap: 10px 22px; margin-top: 14px; max-width: 920px; font-family: var(--mono); font-size: .72rem; letter-spacing: .06em; color: var(--pencil); }
.video-meta strong { font-family: var(--serif); font-size: 1rem; font-weight: 600; letter-spacing: 0; color: var(--paper-bright); }
.video-meta a { color: var(--paper); text-transform: uppercase; letter-spacing: .1em; display: inline-flex; gap: 6px; align-items: center; }
#panel-video .label-head { margin-top: 38px; max-width: 920px; }
.video-list { max-width: 920px; border-top: 1px solid var(--rule); }
.video-list a { display: grid; grid-template-columns: 3em minmax(0, 1fr) auto; gap: 12px; align-items: baseline; padding: 13px 0; border-bottom: 1px solid var(--rule); color: var(--paper); text-decoration: none; }
.video-list .vn { font-family: var(--mono); font-size: .76rem; color: var(--accent); }
.video-list small { display: block; color: var(--pencil); font-size: .82rem; }
.video-list .go { font-family: var(--mono); font-size: .66rem; letter-spacing: .1em; text-transform: uppercase; color: var(--graphite); }
.video-list a:hover { color: var(--accent); }
.video-list a:hover .go { color: var(--accent); }

/* ---------- past papers & mark schemes ---------- */
.pp-toolbar {
  display: flex; flex-wrap: wrap; align-items: center; gap: 10px 22px; margin: 10px 0 28px; padding: 11px 0;
  border-top: 1px solid var(--rule-strong); border-bottom: 1px solid var(--rule-strong);
  font-family: var(--mono); font-size: .72rem; letter-spacing: .08em; text-transform: uppercase; color: var(--pencil);
}
.pp-toolbar label { display: inline-flex; align-items: center; gap: 9px; }
.pp-toolbar .grow { flex: 1; }
.pp-toolbar select { text-transform: none; letter-spacing: .02em; }
.ppq { margin: 0 0 32px; background: var(--ink-raised); border: 1px solid var(--rule-strong); }
.ppq > header { display: flex; flex-wrap: wrap; align-items: baseline; gap: 6px 18px; padding: 12px 18px 11px; border-bottom: 1px solid var(--rule); }
.ppq > header h3 { flex: 1 1 280px; margin: 0; display: flex; flex-wrap: wrap; align-items: baseline; gap: 2px 14px; font-size: 1rem; }
.ppq > header .src { font-family: var(--mono); font-size: .7rem; font-weight: 500; letter-spacing: .1em; text-transform: uppercase; color: var(--pencil); }
.ppq > header .q { font-family: var(--serif); font-size: 1.32rem; font-weight: 600; color: var(--paper-bright); font-variation-settings: "opsz" 48; }
.ppq .marks { font-family: var(--mono); font-size: .76rem; color: var(--accent); }
.ppq .marks::before { content: "["; } .ppq .marks::after { content: "]"; }
.ppq .type { font-family: var(--mono); font-size: .66rem; letter-spacing: .12em; text-transform: uppercase; color: var(--graphite); }
.ppq .done-toggle { display: inline-flex; align-items: center; gap: 9px; cursor: pointer; font-family: var(--mono); font-size: .7rem; letter-spacing: .1em; text-transform: uppercase; color: var(--pencil); }
.ppq .done-toggle input:checked { background: var(--good); border-color: var(--good); }
.ppq.is-done { border-color: color-mix(in srgb, var(--good) 45%, transparent); }
.ppq.is-done > header .q::after { content: " ✓"; color: var(--good); }
/* scans are inverted to sit on the dark page; "White paper" shows them as printed */
.ppq .imgs { padding: 18px 12px; background: #201f1e; }
.ppq .imgs img { display: block; width: 100%; max-width: 780px; height: auto; margin: 0 auto; cursor: zoom-in; filter: invert(.88) sepia(.12) contrast(.97); }
.ppq .imgs img + img { margin-top: 12px; padding-top: 12px; border-top: 1px dashed rgba(236, 228, 214, .18); }
.ms .imgs img { max-width: 100%; }
body.paper-white .ppq .imgs { background: #fff; }
body.paper-white .ppq .imgs img { filter: none; }
body.paper-white .ppq .imgs img + img { border-top-color: #c9c3b8; }
.ppq footer { display: flex; flex-wrap: wrap; justify-content: space-between; gap: 8px; padding: 10px 18px; border-top: 1px solid var(--rule); font-family: var(--mono); font-size: .7rem; letter-spacing: .06em; color: var(--graphite); }
.ppq footer a { color: var(--accent); text-decoration: none; text-transform: uppercase; letter-spacing: .1em; }
.ppq footer a:hover { text-decoration: underline; }
.mcq-key { display: flex; align-items: center; gap: 14px; padding: 16px 18px; font-family: var(--mono); font-size: .74rem; letter-spacing: .1em; text-transform: uppercase; color: var(--pencil); }
.mcq-key b { display: inline-grid; place-items: center; width: 44px; height: 44px; border: 1.5px solid var(--good); font-family: var(--serif); font-size: 1.5rem; letter-spacing: 0; color: var(--good); }
.empty { padding: 26px; text-align: center; color: var(--pencil); border: 1px dashed var(--rule-strong); font-family: var(--mono); font-size: .8rem; letter-spacing: .04em; }
.ms-warning { font-size: .88rem; color: var(--pencil); border-left: 2px solid var(--u2); padding: 8px 14px; margin-bottom: 16px; }

/* ---------- practice questions ---------- */
.score-bar { display: flex; flex-wrap: wrap; align-items: center; gap: 12px; margin-bottom: 10px; padding: 11px 0; border-top: 1px solid var(--rule-strong); border-bottom: 1px solid var(--rule-strong); font-family: var(--mono); font-size: .72rem; letter-spacing: .08em; text-transform: uppercase; color: var(--pencil); }
.pq { position: relative; padding: 22px 0 20px 70px; border-bottom: 1px solid var(--rule); }
.pq-head { display: flex; align-items: baseline; gap: 12px; margin-bottom: 2px; }
.pq-num { position: absolute; left: 0; top: 14px; font-family: var(--serif); font-size: 2.1rem; font-weight: 600; line-height: 1; color: var(--accent); font-variation-settings: "opsz" 144, "SOFT" 80, "WONK" 1; }
.pq-marks { font-family: var(--mono); font-size: .74rem; color: var(--pencil); }
.pq-marks::before { content: "["; } .pq-marks::after { content: "]"; }
.pq-kind { margin-left: auto; font-family: var(--mono); font-size: .66rem; letter-spacing: .12em; text-transform: uppercase; color: var(--graphite); }
.pq-body > :first-child { margin-top: .2em; }
.mcq-opts { display: grid; gap: 7px; margin: 12px 0 6px; }
.mcq-opt { display: flex; align-items: center; gap: 14px; width: 100%; padding: 9px 14px; text-align: left; cursor: pointer; background: transparent; border: 1px solid var(--rule); color: var(--paper); font: inherit; }
.mcq-opt:hover { background: var(--ink-raised); border-color: var(--rule-strong); }
.mcq-opt .l { display: inline-grid; place-items: center; flex: none; width: 26px; height: 26px; border: 1px solid var(--rule-strong); font-family: var(--mono); font-size: .76rem; }
.mcq-opt.right { border-color: var(--good); }
.mcq-opt.right .l { background: var(--good); border-color: var(--good); color: var(--ink); }
.mcq-opt.wrong { border-color: var(--bad); }
.mcq-opt.wrong .l { background: var(--bad); border-color: var(--bad); color: var(--ink); }
.mcq-feedback { min-height: 1.4em; font-family: var(--mono); font-size: .74rem; letter-spacing: .06em; }
.mcq-feedback.ok { color: var(--good); }
.mcq-feedback.no { color: var(--bad); }
.pq details { margin-top: 10px; }
.pq summary { display: inline-flex; align-items: center; gap: 8px; padding: 4px 0; cursor: pointer; list-style: none; font-family: var(--mono); font-size: .72rem; letter-spacing: .12em; text-transform: uppercase; color: var(--accent); }
.pq summary::-webkit-details-marker { display: none; }
.pq summary::before { content: "+"; display: inline-grid; place-items: center; width: 16px; height: 16px; border: 1px solid currentColor; font-size: .8rem; line-height: 1; }
.pq details[open] summary::before { content: "−"; }
.pq .ans { margin-top: 10px; padding: 4px 0 4px 18px; border-left: 2px solid var(--good); }

/* ---------- chapter pager ---------- */
.pager { display: grid; grid-template-columns: minmax(0, 1fr) minmax(0, 1fr); margin: 42px 0 10px; border-top: 1px solid var(--rule-strong); border-bottom: 1px solid var(--rule-strong); }
.pager a { display: block; padding: 18px 18px; color: var(--paper-bright); text-decoration: none; font-family: var(--serif); font-size: 1.15rem; line-height: 1.3; font-variation-settings: "opsz" 36; }
.pager a.next { grid-column: 2; text-align: right; border-left: 1px solid var(--rule); }
.pager a:hover { color: var(--accent); }
.pager small { display: block; margin-bottom: 5px; font-family: var(--mono); font-size: .68rem; letter-spacing: .14em; text-transform: uppercase; color: var(--pencil); }

/* ---------- lightbox ---------- */
.lightbox { position: fixed; inset: 0; z-index: 90; display: none; overflow: auto; padding: 60px 12px 24px; background: rgba(10, 9, 8, .95); }
.lightbox.open { display: block; }
.lightbox img { display: block; margin: 0 auto; max-width: min(1400px, 100%); height: auto; background: #fff; filter: invert(.88) sepia(.12) contrast(.97); }
body.paper-white .lightbox img { filter: none; }
.lightbox button { position: fixed; top: 12px; right: 14px; }

.flash { animation: flash 1.4s ease; }
@keyframes flash { 0%, 45% { outline: 2px solid var(--accent); outline-offset: 4px; } 100% { outline: 2px solid transparent; outline-offset: 12px; } }

/* ---------- footer ---------- */
.site-footer { margin-top: 80px; padding: 34px 0 46px; border-top: 1px solid var(--rule-strong); color: var(--pencil); font-size: .88rem; }
.site-footer .cols { display: flex; flex-wrap: wrap; justify-content: space-between; gap: 20px 40px; }
.site-footer p { margin: .35em 0; max-width: 68ch; }
.site-footer a { color: var(--paper); }
.foot-mark { display: flex; align-items: center; gap: 14px; color: var(--paper); }
.foot-mark .tile { width: 34px; height: 34px; font-size: 16px; }
.foot-links { font-family: var(--mono); font-size: .72rem; letter-spacing: .1em; text-transform: uppercase; }
.colophon { margin-top: 1em !important; color: var(--graphite); font-size: .66rem; line-height: 1.7; }

/* ---------- responsive ---------- */
@media (max-width: 1060px) {
  .notes-layout { grid-template-columns: minmax(0, 1fr); }
  .toc { display: none; }
}
@media (max-width: 980px) {
  .two-col { grid-template-columns: minmax(0, 1fr); gap: 34px; }
}
@media (max-width: 900px) {
  .hero { grid-template-columns: minmax(0, 1fr); gap: 34px; padding-top: 44px; }
  .unit-index a { grid-template-columns: 110px minmax(0, 1fr); }
  .unit-index .meta { grid-column: 2; flex-direction: row; flex-wrap: wrap; gap: 4px 16px; }
  .sim-body { grid-template-columns: minmax(0, 1fr); }
  .sim-side { border-left: 0; border-top: 1px solid var(--rule); }
}
@media (max-width: 720px) {
  /* phones: the header scrolls away, the nav gets its own row, the tab bar sticks to the top */
  :root { --header-h: 0px; }
  .site-header { position: relative; }
  .site-header .wrap { flex-wrap: wrap; height: auto; padding-top: 10px; padding-bottom: 4px; column-gap: 12px; row-gap: 6px; }
  .brand { flex: 1 1 auto; min-width: 0; }
  .main-nav { order: 3; flex: 0 0 100%; margin-left: 0; justify-content: space-between; gap: 0; border-top: 1px solid var(--rule); }
  .main-nav a { padding: 10px 4px 9px; font-size: .68rem; letter-spacing: .06em; gap: 6px; }
  .wrap { padding: 0 16px; }
  .section-head { grid-template-columns: minmax(0, 1fr); margin-top: 56px; }
  .section-head p { grid-column: 1; }
  .unit-index a { grid-template-columns: 64px minmax(0, 1fr); gap: 8px 18px; padding: 22px 0; }
  .unit-index .no { font-size: 3.2rem; }
  .unit-index .t { font-size: 1.35rem; }
  .inside { grid-template-columns: minmax(0, 1fr); }
  .unit-head, .chapter-head { grid-template-columns: minmax(0, 1fr); gap: 0; }
  .big-no { font-size: 4.6rem; }
  .ch-no { font-size: 3.6rem; }
  .chapter-list a { flex-wrap: wrap; gap: 4px 14px; }
  .chapter-list .dots { display: none; }
  .chapter-list .m { flex-basis: 100%; padding-left: calc(3.3em + 14px); }
  .chapter-list .done { padding-left: calc(3.3em + 14px); }
  .pq { padding-left: 0; }
  .pq-num { position: static; font-size: 1.6rem; }
  .spectro-body { height: 440px; }
  .hero h1 { max-width: none; }
}
@media (max-width: 460px) {
  body { font-size: 16px; }
  .wordmark small { display: none; }
  .ppq > header { padding: 10px 12px; }
  .ppq .imgs { padding: 10px 4px; }
  .sim-head, .sim-caption { padding-left: 12px; padding-right: 12px; }
}
@media print {
  .site-header, .tabbar, .site-footer, .pp-toolbar, .toc, .pager { display: none !important; }
  .panel[hidden] { display: block !important; }
  body, html { background: #fff !important; color: #000 !important; }
  body::before { display: none; }
  .ppq .imgs img, .lightbox img { filter: none !important; }
  .ppq .imgs { background: #fff !important; }
}
@media (prefers-reduced-motion: reduce) { * { transition: none !important; animation: none !important; } }