:root {
  --bg: #fbfaf7;
  --bg-elevated: #ffffff;
  --ink: #15212e;
  --ink-soft: #3a4a5c;
  --ink-muted: #6b7785;
  --rule: #e3e0d8;
  --upstream: #1f3a52;
  --midmarket: #4a7396;
  --downstream: #7aa6cc;
  --feedback: #16a085;
  --accent: #c8392c;
  --gold: #c9a227;
  --cream: #f6f1e4;
  --serif: 'Newsreader', Georgia, serif;
  --sans: 'Inter', system-ui, sans-serif;
  --mono: 'JetBrains Mono', ui-monospace, monospace;
}
* { box-sizing: border-box; }
html { scroll-behavior: smooth; }
body {
  margin: 0; background: var(--bg); color: var(--ink);
  font-family: var(--serif); font-size: 19px; line-height: 1.62;
  -webkit-font-smoothing: antialiased;
}
a { color: var(--upstream); }
em { font-style: italic; }
strong { font-weight: 600; }
code { font-family: var(--mono); font-size: .85em; background: #f0ede4; padding: .05em .3em; border-radius: 3px; }

/* ---------- top nav ---------- */
.topnav {
  position: fixed; top: 0; left: 0; right: 0; height: 52px; z-index: 50;
  background: rgba(251, 250, 247, .94); backdrop-filter: blur(8px);
  border-bottom: 1px solid var(--rule);
  display: flex; align-items: center; justify-content: space-between; padding: 0 1.4rem;
  font-family: var(--sans); font-size: .82rem;
}
.topnav .brand { font-weight: 600; color: var(--ink); text-decoration: none; letter-spacing: .01em; }
.topnav .brand em { color: var(--accent); font-family: var(--serif); font-size: 1.05rem; }
.topnav .meta { color: var(--ink-muted); }
.topnav .meta a { color: var(--ink-muted); text-decoration: none; border-bottom: 1px dotted var(--ink-muted); }
.progress { position: absolute; left: 0; bottom: -1px; height: 2px; background: var(--accent); width: 0; transition: width .08s linear; }

/* ---------- chapter rail ---------- */
.rail { position: fixed; left: 1.6rem; top: 50%; transform: translateY(-50%); z-index: 40;
  display: flex; flex-direction: column; gap: .9rem; font-family: var(--sans); }
.rail a { display: flex; align-items: center; gap: .6rem; text-decoration: none; color: var(--ink-muted); font-size: .72rem; }
.rail .dot { width: 9px; height: 9px; border-radius: 50%; border: 1.5px solid var(--ink-muted); background: transparent; transition: all .2s; flex-shrink: 0; }
.rail .lbl { opacity: 0; transform: translateX(-4px); transition: all .2s; white-space: nowrap; }
.rail a:hover .lbl, .rail a.active .lbl { opacity: 1; transform: none; }
.rail a.active .dot { background: var(--accent); border-color: var(--accent); transform: scale(1.25); }
.rail a.active { color: var(--accent); }
@media (max-width: 1100px) { .rail { display: none; } }

/* ---------- chapters ---------- */
.chapter { max-width: 920px; margin: 0 auto; padding: 7rem 1.5rem 3rem; border-bottom: 1px solid var(--rule); }
.chapter:last-of-type { border-bottom: none; }
.ch-num { font-family: var(--sans); font-size: .74rem; letter-spacing: .16em; text-transform: uppercase; color: var(--accent); font-weight: 600; }
.chapter h2 { font-size: 2.9rem; line-height: 1.08; margin: .4rem 0 1rem; font-weight: 500; letter-spacing: -.01em; }
.lede { font-size: 1.3rem; font-style: italic; color: var(--ink-soft); line-height: 1.5; margin: 0 0 2rem; }
.chapter h3 { font-family: var(--sans); font-size: 1rem; text-transform: uppercase; letter-spacing: .08em; color: var(--upstream); margin: 2.4rem 0 .6rem; }
.chapter p { margin: 0 0 1.1rem; }

/* ---------- cover ---------- */
.cover { padding-top: 8.5rem; }
.cover-eyebrow { font-family: var(--sans); font-size: .78rem; letter-spacing: .2em; text-transform: uppercase; color: var(--accent); font-weight: 600; }
.cover-title { font-size: 4rem; line-height: 1.02; font-weight: 500; letter-spacing: -.02em; margin: .8rem 0 .6rem; }
.cover-title em { color: var(--accent); }
.cover-sub { font-size: 1.4rem; color: var(--ink-soft); font-style: italic; margin-bottom: 1.4rem; }
.cover-byline { font-family: var(--sans); font-size: .86rem; color: var(--ink-soft); line-height: 1.6; margin-bottom: 2.4rem; }
.cover-byline .muted { color: var(--ink-muted); }
.cover-abstract, .plain-card {
  background: var(--cream); border-left: 4px solid var(--gold); padding: 1.4rem 1.7rem 0.5rem; margin: 1.6rem 0 2rem; border-radius: 0 6px 6px 0;
}
.cover-abstract p { font-size: 1.08rem; }
.cover-abstract-label, .plain-label { font-family: var(--sans); font-size: .72rem; letter-spacing: .16em; text-transform: uppercase; color: #8a6d12; font-weight: 700; margin-bottom: .7rem; }
.plain-card p { font-size: 1.02rem; }
.cover-pillars { display: grid; grid-template-columns: repeat(3, 1fr); gap: 1rem; margin-top: 2rem; }
.pillar { background: var(--bg-elevated); border: 1px solid var(--rule); border-top: 3px solid var(--upstream); padding: 1.1rem 1.2rem; border-radius: 4px; }
.pillar-num { font-family: var(--serif); font-style: italic; color: var(--accent); font-size: 1.5rem; }
.pillar h4 { font-family: var(--sans); font-size: .95rem; margin: .2rem 0 .4rem; }
.pillar p { font-size: .92rem; color: var(--ink-soft); margin: 0; line-height: 1.5; }
@media (max-width: 760px) { .cover-pillars { grid-template-columns: 1fr; } .cover-title { font-size: 2.6rem; } .chapter h2 { font-size: 2.1rem; } }

/* ---------- math ---------- */
.equation-block { background: var(--bg-elevated); border: 1px solid var(--rule); border-radius: 6px; padding: .9rem 1.2rem; margin: 1.2rem 0 1.4rem; overflow-x: auto; }
.equation-block .eq-tag { font-family: var(--sans); font-size: .7rem; color: var(--ink-muted); letter-spacing: .1em; text-transform: uppercase; }
.equation-block .katex-display { margin: .5rem 0; }

/* ---------- interactive ---------- */
.interactive-section { background: var(--bg-elevated); border: 1px solid var(--rule); border-radius: 8px; padding: 1.3rem 1.3rem 1rem; margin: 1.8rem 0; box-shadow: 0 1px 2px rgba(21, 33, 46, .04); }
.interactive-title { font-family: var(--sans); font-weight: 600; font-size: .95rem; margin-bottom: .25rem; }
.hint { font-family: var(--sans); font-size: .76rem; color: var(--ink-muted); margin-bottom: 1rem; }
.hint::before { content: '●'; color: var(--feedback); margin-right: .4rem; font-size: .6rem; vertical-align: middle; }
.controls { display: grid; grid-template-columns: repeat(auto-fit, minmax(200px, 1fr)); gap: .8rem 1.4rem; margin-bottom: 1rem; font-family: var(--sans); }
.ctrl label { display: flex; justify-content: space-between; font-size: .8rem; color: var(--ink-soft); margin-bottom: .2rem; }
.ctrl label .val { font-family: var(--mono); color: var(--accent); font-weight: 500; }
.ctrl input[type=range] { width: 100%; accent-color: var(--upstream); }
.ctrl select { width: 100%; font-family: var(--sans); font-size: .85rem; padding: .3rem; border: 1px solid var(--rule); border-radius: 4px; background: #fff; }
.chart-grid { display: grid; grid-template-columns: 1fr 1fr; gap: .8rem; }
.chart-grid.one { grid-template-columns: 1fr; }
.chart { min-height: 330px; width: 100%; }
.chart.tall { min-height: 420px; }
@media (max-width: 760px) { .chart-grid { grid-template-columns: 1fr; } }
.readouts { display: grid; grid-template-columns: repeat(auto-fit, minmax(140px, 1fr)); gap: .6rem; margin-top: .9rem; }
.readout { background: var(--bg); border: 1px solid var(--rule); border-radius: 5px; padding: .55rem .75rem; font-family: var(--sans); }
.readout .k { font-size: .68rem; text-transform: uppercase; letter-spacing: .08em; color: var(--ink-muted); }
.readout .v { font-family: var(--mono); font-size: 1.15rem; color: var(--ink); }
.readout .v.good { color: var(--feedback); }
.readout .v.bad { color: var(--accent); }
.btn-row { display: flex; flex-wrap: wrap; gap: .5rem; margin: .2rem 0 1rem; font-family: var(--sans); }
.btn { font-family: var(--sans); font-size: .8rem; padding: .42rem .85rem; border: 1px solid var(--upstream); background: #fff; color: var(--upstream); border-radius: 4px; cursor: pointer; transition: all .15s; }
.btn:hover { background: var(--upstream); color: #fff; }
.btn.active { background: var(--upstream); color: #fff; }
.btn.ghost { border-color: var(--rule); color: var(--ink-soft); }
.cards { display: grid; grid-template-columns: repeat(auto-fit, minmax(150px, 1fr)); gap: .6rem; margin-bottom: 1rem; font-family: var(--sans); }
.card-opt { border: 1px solid var(--rule); border-radius: 5px; padding: .6rem .75rem; cursor: pointer; background: #fff; transition: all .15s; }
.card-opt:hover { border-color: var(--midmarket); }
.card-opt.active { border-color: var(--accent); box-shadow: inset 3px 0 0 var(--accent); }
.card-opt .t { font-weight: 600; font-size: .85rem; }
.card-opt .d { font-size: .74rem; color: var(--ink-muted); line-height: 1.35; margin-top: .2rem; }
.mask-wrap { display: flex; gap: 1.2rem; align-items: flex-start; flex-wrap: wrap; }
.mask-wrap svg { flex-shrink: 0; }
.mask-legend { font-family: var(--sans); font-size: .8rem; color: var(--ink-soft); max-width: 330px; }
.mask-legend .sw { display: inline-block; width: 12px; height: 12px; vertical-align: middle; margin-right: .4rem; border: 1px solid #bbb; }
.detail-panel { font-family: var(--sans); font-size: .86rem; background: var(--bg); border: 1px dashed var(--rule); padding: .7rem .9rem; border-radius: 5px; margin-top: .8rem; min-height: 3rem; color: var(--ink-soft); }

/* ---------- callouts, tables ---------- */
.callout { border-left: 4px solid var(--accent); background: #fdf3f1; padding: 1rem 1.4rem; margin: 2rem 0 1.5rem; border-radius: 0 6px 6px 0; }
.callout .callout-label { font-family: var(--sans); font-size: .72rem; letter-spacing: .16em; text-transform: uppercase; color: var(--accent); font-weight: 700; margin-bottom: .35rem; }
.callout p { margin: 0; }
table.data { width: 100%; border-collapse: collapse; font-family: var(--sans); font-size: .82rem; margin: 1rem 0 1.4rem; }
table.data th, table.data td { padding: .38rem .5rem; border-bottom: 1px solid var(--rule); text-align: right; }
table.data th:first-child, table.data td:first-child { text-align: left; }
table.data th { font-weight: 600; color: var(--ink-soft); font-size: .74rem; text-transform: uppercase; letter-spacing: .04em; }
table.data td.num { font-family: var(--mono); }
table.data tr.hl td { background: #fbf6e6; font-weight: 600; }
.caption { font-family: var(--sans); font-size: .76rem; color: var(--ink-muted); margin-top: -.6rem; margin-bottom: 1.2rem; }

.chapter-foot { display: flex; justify-content: flex-end; align-items: center; gap: .6rem; margin-top: 2.6rem; font-family: var(--sans); font-size: .9rem; color: var(--ink-soft); text-decoration: none; }
.chapter-foot .arrow { transition: transform .2s; display: inline-block; }
.chapter-foot:hover { color: var(--accent); }
.chapter-foot:hover .arrow { transform: translateX(6px); }
footer { max-width: 920px; margin: 0 auto; padding: 2rem 1.5rem 4rem; font-family: var(--sans); font-size: .78rem; color: var(--ink-muted); }