:root {
  --paper: #f4f0e8;
  --paper-deep: #e8e1d4;
  --ink: #13252a;
  --muted: #637175;
  --line: rgba(19, 37, 42, 0.17);
  --white: #fffdf8;
  --accent: #e95f3b;
  --accent-soft: #ffd4c6;
  --accent-2: #245f68;
  --dark: #10282d;
  --serif: Iowan Old Style, Baskerville, "Noto Serif SC", "Songti SC", SimSun, serif;
  --sans: Inter, ui-sans-serif, system-ui, -apple-system, "PingFang SC", "Microsoft YaHei", sans-serif;
}

body.theme-video {
  --accent: #e05b3f;
  --accent-soft: #ffd2c4;
  --accent-2: #3d6f8f;
  --dark: #172c38;
}

body.theme-graph {
  --accent: #df3f44;
  --accent-soft: #ffd0d1;
  --accent-2: #3c4d91;
  --dark: #191f35;
}

body.theme-shell {
  --accent: #6955c8;
  --accent-soft: #ddd7ff;
  --accent-2: #2b7f79;
  --dark: #1e2038;
}

* { box-sizing: border-box; }
html { scroll-behavior: smooth; }
body {
  margin: 0;
  color: var(--ink);
  background: var(--paper);
  font-family: var(--sans);
  line-height: 1.7;
}

a { color: inherit; }
button, input { font: inherit; }
button { color: inherit; }
svg { display: block; max-width: 100%; }
.reading-progress { position: fixed; z-index: 100; inset: 0 0 auto; height: 3px; background: rgba(255, 255, 255, .2); }
.reading-progress span { display: block; width: 0; height: 100%; background: var(--accent); }

.topbar {
  position: sticky;
  z-index: 50;
  top: 0;
  min-height: 60px;
  padding: 0 clamp(18px, 4vw, 58px);
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 24px;
  color: var(--ink);
  background: color-mix(in srgb, var(--paper) 92%, transparent);
  border-bottom: 1px solid var(--line);
  backdrop-filter: blur(16px);
}

.brand { display: flex; align-items: center; gap: 10px; font-size: 11px; font-weight: 800; letter-spacing: .1em; text-decoration: none; text-transform: uppercase; }
.brand-mark { display: grid; place-items: center; width: 32px; height: 32px; color: var(--paper); background: var(--dark); border-radius: 50%; font: 700 15px/1 var(--serif); letter-spacing: 0; }
.topbar nav { display: flex; gap: 22px; }
.topbar nav a { color: var(--muted); font-size: 11px; font-weight: 750; text-decoration: none; }
.topbar nav a:hover, .topbar nav a:focus-visible { color: var(--accent); }
.topbar-actions { display: flex; align-items: center; gap: 8px; }
.paper-link { padding: 8px 12px; color: var(--paper); background: var(--dark); border-radius: 4px; font-size: 10px; font-weight: 800; text-decoration: none; }
.paper-link.secondary { color: var(--ink); background: transparent; border: 1px solid var(--line); }

.hero { min-height: calc(100vh - 60px); padding: clamp(54px, 8vw, 110px) clamp(22px, 7vw, 110px) 56px; overflow: hidden; }
.hero-grid { min-height: 640px; display: grid; grid-template-columns: minmax(0, .95fr) minmax(420px, 1.05fr); align-items: center; gap: clamp(38px, 7vw, 112px); }
.eyebrow, .kicker { margin: 0 0 20px; color: var(--accent); font-size: 10px; font-weight: 850; letter-spacing: .16em; text-transform: uppercase; }
h1, h2, h3, p { margin-top: 0; }
h1 { max-width: 860px; margin-bottom: 26px; font: 700 clamp(46px, 5.8vw, 88px)/.94 var(--serif); letter-spacing: -.055em; }
h1 em { color: var(--accent); font-weight: 400; }
h1 span { display: block; margin-top: 22px; color: var(--muted); font: 650 clamp(17px, 1.5vw, 24px)/1.35 var(--sans); letter-spacing: -.02em; }
.hero-deck { max-width: 720px; color: #394b50; font-size: clamp(16px, 1.5vw, 21px); line-height: 1.8; }
.hero-deck strong { color: var(--ink); }
.hero-actions { display: flex; flex-wrap: wrap; align-items: center; gap: 12px; margin-top: 32px; }
.hero-actions > span { color: var(--muted); font-size: 11px; }
.primary-button, .ghost-button { padding: 11px 16px; border-radius: 4px; font-size: 11px; font-weight: 800; text-decoration: none; }
.primary-button { color: var(--paper); background: var(--dark); }
.ghost-button { border: 1px solid var(--line); }
.primary-button:hover, .ghost-button:hover, .paper-link:hover { transform: translateY(-2px); }
.hero-stage { position: relative; min-height: 540px; padding: 24px; display: grid; place-items: center; background: var(--white); border: 1px solid var(--line); border-radius: 20px; box-shadow: 0 28px 70px rgba(19, 37, 42, .12); }
.hero-stage > svg { width: 100%; max-height: 500px; }
.hero-stage .stage-note { position: absolute; right: 22px; bottom: 16px; margin: 0; color: var(--muted); font-size: 9px; letter-spacing: .08em; }
.hero-facts { display: grid; grid-template-columns: repeat(4, 1fr); gap: 0; margin-top: 40px; border-top: 1px solid var(--line); border-bottom: 1px solid var(--line); }
.hero-facts div { min-height: 120px; padding: 20px; border-right: 1px solid var(--line); }
.hero-facts div:last-child { border-right: 0; }
.hero-facts strong { display: block; color: var(--accent); font: 700 clamp(28px, 3vw, 46px)/1 var(--serif); }
.hero-facts span { display: block; margin-top: 10px; color: var(--muted); font-size: 11px; }

.chapter { position: relative; padding: clamp(82px, 10vw, 150px) clamp(22px, 7vw, 110px); border-top: 1px solid var(--line); }
.chapter.dark { color: var(--paper); background: var(--dark); }
.chapter.tint { background: var(--paper-deep); }
.section-marker { display: grid; grid-template-columns: 56px 1fr; gap: 14px; align-items: center; margin-bottom: 54px; }
.section-marker span { color: var(--accent); font: italic 700 30px/1 var(--serif); }
.section-marker p { margin: 0; color: var(--muted); font-size: 10px; font-weight: 850; letter-spacing: .16em; }
.dark .section-marker p { color: rgba(255, 255, 255, .55); }
.chapter-copy { max-width: 900px; }
.chapter-copy h2, .chapter-heading h2 { margin-bottom: 24px; font: 700 clamp(38px, 5vw, 72px)/1.04 var(--serif); letter-spacing: -.045em; }
.chapter-copy .lead, .chapter-heading > p:last-child { color: #3e5054; font-size: clamp(16px, 1.4vw, 20px); }
.dark .chapter-copy .lead, .dark .chapter-heading > p:last-child { color: rgba(255, 255, 255, .72); }
.chapter-copy > p:not(.kicker):not(.lead) { max-width: 760px; color: var(--muted); }

.principle-strip { display: grid; grid-template-columns: repeat(3, 1fr); gap: 14px; margin-top: 52px; }
.principle-card { min-height: 210px; padding: 24px; background: var(--white); border: 1px solid var(--line); border-radius: 8px; }
.principle-card > span { color: var(--accent); font: italic 700 26px var(--serif); }
.principle-card h3 { margin: 28px 0 12px; font: 700 24px/1.2 var(--serif); }
.principle-card p { margin: 0; color: var(--muted); font-size: 12px; }
.dark .principle-card { background: rgba(255, 255, 255, .055); border-color: rgba(255, 255, 255, .16); }
.dark .principle-card p { color: rgba(255, 255, 255, .65); }

.visual-panel { margin: 52px 0 0; padding: clamp(18px, 3vw, 34px); background: var(--white); border: 1px solid var(--line); border-radius: 12px; }
.dark .visual-panel { background: rgba(255, 255, 255, .045); border-color: rgba(255, 255, 255, .16); }
.visual-panel svg { width: 100%; height: auto; }
.visual-caption { display: flex; justify-content: space-between; gap: 20px; margin-top: 18px; padding-top: 14px; border-top: 1px solid var(--line); color: var(--muted); font-size: 10px; }
.dark .visual-caption { border-color: rgba(255, 255, 255, .14); color: rgba(255, 255, 255, .54); }

.process-shell { display: grid; grid-template-columns: minmax(280px, .78fr) minmax(0, 1.22fr); gap: 18px; margin-top: 52px; }
.process-tabs { display: grid; gap: 8px; align-content: start; }
.process-tab { padding: 16px 18px; display: grid; grid-template-columns: 38px 1fr; gap: 10px; text-align: left; background: transparent; border: 1px solid rgba(255, 255, 255, .18); border-radius: 6px; cursor: pointer; }
.process-tab span { color: var(--accent); font: italic 700 18px var(--serif); }
.process-tab strong { font-size: 13px; }
.process-tab small { grid-column: 2; color: rgba(255, 255, 255, .52); font-size: 10px; }
.process-tab.active { color: var(--dark); background: var(--accent-soft); border-color: var(--accent-soft); }
.process-tab.active small { color: rgba(16, 40, 45, .6); }
.process-display { min-height: 390px; padding: clamp(24px, 4vw, 46px); background: rgba(255, 255, 255, .06); border: 1px solid rgba(255, 255, 255, .16); border-radius: 10px; }
.process-copy-count { color: var(--accent); font-size: 10px; font-weight: 850; letter-spacing: .12em; }
.process-copy h3 { margin: 18px 0; font: 700 clamp(28px, 3vw, 44px)/1.08 var(--serif); }
.process-copy p { max-width: 670px; color: rgba(255, 255, 255, .72); }
.process-diagram { min-height: 150px; margin-top: 28px; display: grid; place-items: center; }
.process-diagram svg { max-height: 190px; }

.curve-lab { display: grid; grid-template-columns: minmax(250px, .75fr) minmax(0, 1.25fr); gap: 22px; margin-top: 50px; }
.curve-controls { padding: 26px; background: var(--white); border: 1px solid var(--line); border-radius: 10px; }
.curve-controls h3 { font: 700 30px/1.1 var(--serif); }
.curve-controls > p { color: var(--muted); font-size: 12px; }
.curve-choices { display: grid; gap: 7px; margin-top: 24px; }
.curve-choice { padding: 11px 13px; text-align: left; color: var(--muted); background: transparent; border: 1px solid var(--line); border-radius: 5px; cursor: pointer; }
.curve-choice.active { color: var(--paper); background: var(--dark); border-color: var(--dark); }
.curve-note { min-height: 84px; margin: 22px 0 0; padding-top: 18px; border-top: 1px solid var(--line); color: var(--ink) !important; }
.curve-chart { padding: 22px; background: var(--white); border: 1px solid var(--line); border-radius: 10px; }
.curve-chart svg { width: 100%; }
.chart-axis { fill: none; stroke: rgba(19, 37, 42, .45); stroke-width: 1.5; }
.chart-grid-line { stroke: rgba(19, 37, 42, .1); stroke-width: 1; }
.target-line { fill: none; stroke: var(--accent); stroke-width: 5; stroke-linecap: round; stroke-linejoin: round; }
.model-line { fill: none; stroke: var(--accent-2); stroke-width: 3; stroke-dasharray: 8 7; stroke-linecap: round; }
.chart-label { fill: var(--muted); font-size: 12px; font-family: var(--sans); }
.chart-legend { display: flex; gap: 18px; margin-top: 12px; color: var(--muted); font-size: 10px; }
.chart-legend i { display: inline-block; width: 24px; height: 3px; margin-right: 6px; vertical-align: middle; background: var(--accent); }
.chart-legend span:last-child i { background: var(--accent-2); }

.strain-lab { margin-top: 44px; padding: 26px; display: grid; grid-template-columns: minmax(240px, .72fr) minmax(0, 1.28fr); gap: 28px; background: var(--white); border: 1px solid var(--line); border-radius: 10px; }
/* .strain-lab is a --white panel inside .chapter.dark, so an uncoloured heading
   inherits the dark chapter's light text and disappears against the panel. */
.strain-copy h3 { color: var(--ink); font: 700 30px/1.12 var(--serif); }
.strain-copy p { color: var(--muted); font-size: 12px; }
.strain-control { display: grid; grid-template-columns: 1fr auto; gap: 10px; align-items: center; margin-top: 24px; }
.strain-control input { width: 100%; accent-color: var(--accent); }
.strain-readout { min-width: 66px; color: var(--accent); font: 700 22px var(--serif); text-align: right; }
.strain-stage { min-height: 330px; display: grid; place-items: center; background: var(--paper); border-radius: 8px; overflow: hidden; }
.strain-stage svg { width: 100%; max-height: 340px; }
.deformable { transform-box: fill-box; transform-origin: center; transition: transform .22s ease; }
.stress-cell { transition: opacity .22s ease, fill .22s ease; }

.metric-grid { display: grid; grid-template-columns: repeat(4, 1fr); gap: 12px; margin-top: 50px; }
.metric { min-height: 190px; padding: 22px; background: var(--white); border: 1px solid var(--line); border-radius: 8px; }
.metric strong { display: block; color: var(--accent); font: 700 clamp(30px, 3vw, 48px)/1 var(--serif); }
.metric span { display: block; margin-top: 12px; font-weight: 750; }
.metric p { margin: 14px 0 0; color: var(--muted); font-size: 11px; }
.metric .bar { height: 6px; margin-top: 18px; background: var(--paper-deep); border-radius: 99px; overflow: hidden; }
.metric .bar i { display: block; height: 100%; background: var(--accent); }

.claim-grid { display: grid; grid-template-columns: 1fr 1fr; gap: 18px; margin-top: 46px; }
.claim-card { padding: 28px; border: 1px solid rgba(255, 255, 255, .16); border-radius: 8px; }
.claim-card span { color: var(--accent); font-size: 10px; font-weight: 850; letter-spacing: .13em; }
.claim-card h3 { margin: 18px 0 12px; font: 700 28px/1.15 var(--serif); }
.claim-card p { color: rgba(255, 255, 255, .68); font-size: 12px; }
.claim-card.good { background: rgba(255, 255, 255, .05); }
.claim-card.caution { background: color-mix(in srgb, var(--accent) 11%, transparent); }

.constraint-board { display: grid; grid-template-columns: .75fr 1.25fr; gap: 20px; margin-top: 52px; }
.constraint-controls { display: grid; gap: 10px; align-content: start; }
.constraint-toggle { padding: 15px 16px; display: flex; justify-content: space-between; gap: 20px; text-align: left; background: transparent; border: 1px solid var(--line); border-radius: 6px; cursor: pointer; }
.constraint-toggle::after { content: "OFF"; color: var(--muted); font-size: 9px; font-weight: 850; }
.constraint-toggle.active { color: var(--paper); background: var(--dark); border-color: var(--dark); }
.constraint-toggle.active::after { content: "ON"; color: var(--accent-soft); }
.constraint-stage { min-height: 420px; padding: 24px; display: grid; place-items: center; background: var(--white); border: 1px solid var(--line); border-radius: 10px; }
.constraint-stage svg { width: 100%; max-height: 380px; }
.constraint-status { margin: 14px 0 0; color: var(--muted); font-size: 11px; }
.graph-edge { transition: opacity .2s ease, stroke .2s ease; }
.graph-edge.off { opacity: .08; }

.equation-lab { display: grid; grid-template-columns: minmax(260px, .8fr) minmax(0, 1.2fr); gap: 22px; margin-top: 50px; }
.equation-console { padding: 28px; color: var(--paper); background: var(--dark); border-radius: 10px; }
.equation-console h3 { font: 700 30px/1.12 var(--serif); }
.equation-console p { color: rgba(255, 255, 255, .62); font-size: 12px; }
.equation-code { min-height: 110px; margin: 24px 0; padding: 16px; color: var(--accent-soft); background: rgba(255, 255, 255, .06); border: 1px solid rgba(255, 255, 255, .14); border-radius: 6px; font-family: ui-monospace, SFMono-Regular, Menlo, monospace; font-size: 12px; overflow-wrap: anywhere; }
.equation-choices { display: flex; flex-wrap: wrap; gap: 8px; }
.equation-choice { padding: 8px 11px; color: var(--paper); background: transparent; border: 1px solid rgba(255, 255, 255, .2); border-radius: 4px; cursor: pointer; }
.equation-choice.active { color: var(--dark); background: var(--accent-soft); border-color: var(--accent-soft); }
.shell-stage { min-height: 440px; padding: 22px; display: grid; place-items: center; background: var(--white); border: 1px solid var(--line); border-radius: 10px; }
.shell-stage svg { width: 100%; max-height: 410px; }
.shell-layer { transition: opacity .3s ease, transform .3s ease; transform-box: fill-box; transform-origin: center; }
.shell-layer:not(.active) { opacity: 0; transform: scale(.88); }

.source-grid { display: grid; grid-template-columns: repeat(2, 1fr); gap: 12px; margin-top: 46px; }
.source-card { min-height: 150px; padding: 22px; display: flex; flex-direction: column; justify-content: space-between; background: var(--white); border: 1px solid var(--line); border-radius: 7px; text-decoration: none; }
.source-card span { color: var(--accent); font-size: 9px; font-weight: 850; letter-spacing: .12em; }
.source-card strong { margin-top: 18px; font: 700 21px/1.2 var(--serif); }
.source-card small { margin-top: 10px; color: var(--muted); }
.source-card:hover { transform: translateY(-3px); box-shadow: 0 12px 30px rgba(19, 37, 42, .09); }

.series-nav { padding: 36px clamp(22px, 7vw, 110px); display: grid; grid-template-columns: 1fr auto 1fr; gap: 20px; align-items: center; color: rgba(255, 255, 255, .72); background: #0b1d21; }
.series-nav a { font-size: 11px; font-weight: 750; text-decoration: none; }
.series-nav a:last-child { text-align: right; }
.series-nav span { color: rgba(255, 255, 255, .38); font-size: 9px; letter-spacing: .12em; }
.reveal { opacity: 0; transform: translateY(20px); transition: opacity .65s ease, transform .65s ease; }
.reveal.visible { opacity: 1; transform: none; }
:focus-visible { outline: 3px solid var(--accent); outline-offset: 3px; }

@media (max-width: 980px) {
  .hero-grid { grid-template-columns: 1fr; }
  .hero-stage { max-width: 760px; width: 100%; justify-self: center; }
  .hero-facts { grid-template-columns: repeat(2, 1fr); }
  .hero-facts div:nth-child(2) { border-right: 0; }
  .hero-facts div:nth-child(-n + 2) { border-bottom: 1px solid var(--line); }
  .process-shell, .curve-lab, .strain-lab, .constraint-board, .equation-lab { grid-template-columns: 1fr; }
  .process-tabs { grid-template-columns: repeat(2, 1fr); }
  .metric-grid { grid-template-columns: repeat(2, 1fr); }
}

@media (max-width: 700px) {
  .topbar nav, .paper-link.secondary { display: none; }
  .topbar { gap: 10px; }
  .brand > span:last-child { display: none; }
  .hero { padding-top: 40px; }
  .hero-grid { min-height: 0; }
  .hero-stage { min-height: 390px; padding: 10px; }
  .hero-facts { grid-template-columns: 1fr 1fr; }
  .hero-facts div { min-height: 112px; padding: 16px; }
  .principle-strip, .claim-grid, .source-grid { grid-template-columns: 1fr; }
  .process-tabs { grid-template-columns: 1fr; }
  .process-display { min-height: 330px; }
  .curve-chart, .visual-panel { overflow-x: auto; }
  .curve-chart svg { min-width: 620px; }
  .strain-stage { min-height: 260px; }
  .metric-grid { grid-template-columns: 1fr; }
  .series-nav { grid-template-columns: 1fr; }
  .series-nav a:last-child { text-align: left; }
  .series-nav span { order: -1; }
}

@media (prefers-reduced-motion: reduce) {
  html { scroll-behavior: auto; }
  *, *::before, *::after { animation: none !important; transition: none !important; }
  .reveal { opacity: 1; transform: none; }
}