:root {
  --bg: #f5f3ee; --surface: #fffdf9; --sunk: #efebe3; --ink: #1d1b18; --muted: #6b655c; --line: #e2dcd2;
  --accent: #2b4c7e; --accent-soft: #e3e9f4;
  --bad: #a3342a; --bad-soft: #f8e6e2; --good: #2d6a3e; --good-soft: #e3f1e6; --warn: #8a5a00; --warn-soft: #faf0d9;
  --c1: #2b5fb4; --c2: #c0472c; --c3: #2d8a55; --grid: #e6e1d7; --axis: #8d8578;
  --shadow: 0 1px 2px rgba(0,0,0,.04), 0 6px 18px rgba(0,0,0,.05);
}
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --bg: #131312; --surface: #1c1b19; --sunk: #232220; --ink: #ece7df; --muted: #a39b8f; --line: #2f2d29;
    --accent: #93b4e6; --accent-soft: #1f2a3b;
    --bad: #f0907f; --bad-soft: #3a201c; --good: #8fd2a0; --good-soft: #1b3122; --warn: #e9c06b; --warn-soft: #362a12;
    --c1: #7aa7f0; --c2: #f08a6c; --c3: #6fcf96; --grid: #2c2a26; --axis: #7d766b;
    --shadow: 0 1px 2px rgba(0,0,0,.3), 0 6px 18px rgba(0,0,0,.3);
  }
}
:root[data-theme="dark"] {
  --bg: #131312; --surface: #1c1b19; --sunk: #232220; --ink: #ece7df; --muted: #a39b8f; --line: #2f2d29;
  --accent: #93b4e6; --accent-soft: #1f2a3b;
  --bad: #f0907f; --bad-soft: #3a201c; --good: #8fd2a0; --good-soft: #1b3122; --warn: #e9c06b; --warn-soft: #362a12;
  --c1: #7aa7f0; --c2: #f08a6c; --c3: #6fcf96; --grid: #2c2a26; --axis: #7d766b;
  --shadow: 0 1px 2px rgba(0,0,0,.3), 0 6px 18px rgba(0,0,0,.3);
}
* { box-sizing: border-box; }
html { scroll-padding-top: 72px; }
body { margin: 0; background: var(--bg); color: var(--ink); font: 16px/1.65 Inter, system-ui, sans-serif; -webkit-text-size-adjust: 100%; }
a { color: var(--accent); }
h1, h2, h3 { font-family: Fraunces, Georgia, serif; font-weight: 600; letter-spacing: -.01em; line-height: 1.2; }
button { font: inherit; color: inherit; }
.katex { font-size: 1.06em; }
.katex-display { overflow-x: auto; overflow-y: hidden; padding: 2px 0; margin: .6em 0; }

/* top bar */
.top { position: sticky; top: 0; z-index: 20; display: flex; align-items: center; gap: 14px; height: 56px; padding: 0 18px;
  background: color-mix(in srgb, var(--bg) 88%, transparent); backdrop-filter: blur(8px); border-bottom: 1px solid var(--line); }
.top .home { color: var(--muted); text-decoration: none; font-size: .9rem; white-space: nowrap; }
.top .brand { font: 600 1.15rem Fraunces, Georgia, serif; color: var(--ink); text-decoration: none; flex: 1; white-space: nowrap; overflow: hidden; text-overflow: ellipsis; }
.top button { background: none; border: 1px solid var(--line); border-radius: 8px; width: 36px; height: 36px; cursor: pointer; }
.menu { display: none; }

/* layout */
.layout { display: grid; grid-template-columns: 290px minmax(0, 1fr); max-width: 1280px; margin: 0 auto; }
.nav { position: sticky; top: 56px; height: calc(100vh - 56px); overflow-y: auto; padding: 18px 14px 40px 18px; border-right: 1px solid var(--line); }
.main { padding: 32px 40px 80px; min-width: 0; outline: none; }
.main > * { max-width: 820px; }

/* nav */
.search { width: 100%; padding: 9px 12px; border: 1px solid var(--line); border-radius: 10px; background: var(--surface); color: var(--ink); font: inherit; font-size: .92rem; margin-bottom: 12px; }
.results a, .tree a { display: block; text-decoration: none; color: var(--ink); border-radius: 8px; padding: 5px 10px; font-size: .9rem; line-height: 1.35; }
.results a:hover, .tree a:hover { background: var(--sunk); }
.results small { display: block; color: var(--muted); font-size: .75rem; }
.results .none { color: var(--muted); font-size: .88rem; padding: 6px 10px; }
.tree .guide-link { font-weight: 600; margin-bottom: 8px; }
.tree details { margin: 2px 0; }
.tree summary { list-style: none; cursor: pointer; padding: 7px 10px; border-radius: 8px; font-weight: 600; font-size: .9rem; display: flex; gap: 8px; align-items: baseline; }
.tree summary::-webkit-details-marker { display: none; }
.tree summary:hover { background: var(--sunk); }
.tree summary .n { color: var(--muted); font-size: .75rem; min-width: 1.3em; }
.tree summary .p { margin-left: auto; color: var(--muted); font-weight: 500; font-size: .72rem; }
.tree details a { padding-left: 32px; position: relative; }
.tree a.on { background: var(--accent-soft); color: var(--accent); font-weight: 600; }
.tree a.done::after { content: "✓"; position: absolute; left: 14px; color: var(--good); font-size: .8rem; }

/* page furniture */
.crumb { color: var(--muted); font-size: .82rem; letter-spacing: .08em; text-transform: uppercase; font-weight: 600; }
.crumb a { color: inherit; text-decoration: none; }
.main h1 { font-size: clamp(1.8rem, 4vw, 2.5rem); margin: 6px 0 10px; }
.lede { font-size: 1.08rem; color: var(--muted); margin: 0 0 18px; }
.chips { display: flex; flex-wrap: wrap; gap: 6px; margin: 0 0 18px; }
.chip { font-size: .74rem; padding: 2px 9px; border-radius: 99px; background: var(--sunk); color: var(--muted); border: 1px solid var(--line); font-variant-numeric: tabular-nums; }
.done-toggle { display: inline-flex; align-items: center; gap: 8px; font-size: .88rem; color: var(--muted); cursor: pointer; user-select: none; }
.done-toggle input { width: 17px; height: 17px; accent-color: var(--good); }
.jump { position: sticky; top: 56px; z-index: 5; display: flex; gap: 6px; overflow-x: auto; padding: 10px 0; margin: 10px 0 8px; background: var(--bg); border-bottom: 1px solid var(--line); scrollbar-width: none; }
.jump a { white-space: nowrap; text-decoration: none; font-size: .85rem; font-weight: 500; color: var(--muted); padding: 4px 12px; border-radius: 99px; border: 1px solid var(--line); background: var(--surface); }
.jump a:hover { color: var(--accent); border-color: var(--accent); }
section.part { margin-top: 34px; }
section.part > h2 { font-size: 1.5rem; margin: 0 0 4px; display: flex; align-items: baseline; gap: 10px; }
section.part > h2 .ic { font-family: Inter, sans-serif; font-size: 1rem; }
section.part > .sub { color: var(--muted); margin: 0 0 16px; font-size: .95rem; }

.idea { background: var(--accent-soft); border-left: 4px solid var(--accent); border-radius: 0 12px 12px 0; padding: 14px 18px; margin: 8px 0 6px; }
.idea b:first-child { display: block; font-size: .78rem; letter-spacing: .1em; text-transform: uppercase; color: var(--accent); margin-bottom: 2px; }
.idea p { margin: .3em 0; }

ol.steps { list-style: none; counter-reset: s; padding: 0; margin: 0; }
ol.steps > li { counter-increment: s; position: relative; padding: 0 0 18px 46px; }
ol.steps > li::before { content: counter(s); position: absolute; left: 0; top: 1px; width: 30px; height: 30px; border-radius: 50%; background: var(--surface); border: 2px solid var(--accent); color: var(--accent); display: grid; place-items: center; font-weight: 600; font-size: .9rem; }
ol.steps > li:not(:last-child)::after { content: ""; position: absolute; left: 15px; top: 34px; bottom: 4px; width: 2px; background: var(--line); }
ol.steps h3 { font: 600 1.05rem/1.4 Inter, sans-serif; margin: 3px 0 4px; }
ol.steps p { margin: .35em 0; }

.work { background: var(--surface); border: 1px solid var(--line); border-radius: 12px; padding: 12px 16px; margin: 10px 0; box-shadow: var(--shadow); }
.work .t { font-size: .75rem; text-transform: uppercase; letter-spacing: .1em; font-weight: 600; color: var(--muted); }
.work table { border-collapse: collapse; width: 100%; }
.work td { padding: 4px 6px; vertical-align: top; border-bottom: 1px dashed var(--line); }
.work tr:last-child td { border-bottom: 0; }
.work td.why { color: var(--muted); font-size: .88rem; width: 42%; }
.tbl { border-collapse: collapse; margin: 10px 0; font-variant-numeric: tabular-nums; }
.tbl th, .tbl td { border: 1px solid var(--line); padding: 5px 12px; text-align: center; }
.tbl th { background: var(--sunk); font-weight: 600; }
.scroll { overflow-x: auto; }

.err { background: var(--surface); border: 1px solid var(--line); border-radius: 12px; margin: 0 0 14px; overflow: hidden; box-shadow: var(--shadow); }
.err > div { padding: 10px 16px; }
.err .wrong { background: var(--bad-soft); }
.err .wrong b, .err .fix b, .err .why b { font-size: .74rem; letter-spacing: .1em; text-transform: uppercase; display: block; }
.err .wrong b { color: var(--bad); }
.err .why b { color: var(--muted); }
.err .fix { background: var(--good-soft); }
.err .fix b { color: var(--good); }
.err p { margin: .2em 0; }

ul.gotchas { list-style: none; padding: 0; margin: 0; display: grid; gap: 10px; }
ul.gotchas li { background: var(--warn-soft); border-radius: 12px; padding: 10px 16px 10px 44px; position: relative; }
ul.gotchas li::before { content: "!"; position: absolute; left: 14px; top: 10px; width: 20px; height: 20px; border-radius: 50%; background: var(--warn); color: var(--surface); font-weight: 700; font-size: .8rem; display: grid; place-items: center; }

.ex { background: var(--surface); border: 1px solid var(--line); border-radius: 14px; padding: 16px 18px; margin: 0 0 16px; box-shadow: var(--shadow); }
.ex .kind { font-size: .74rem; letter-spacing: .1em; text-transform: uppercase; font-weight: 600; color: var(--accent); }
.ex .ask { margin: 6px 0 10px; font-size: 1.02rem; }
.ex .ask p { margin: .3em 0; }
.ex .btns { display: flex; flex-wrap: wrap; gap: 8px; }
.ex .btns button { border: 1px solid var(--line); background: var(--sunk); border-radius: 99px; padding: 5px 14px; font-size: .86rem; font-weight: 500; cursor: pointer; }
.ex .btns button[aria-expanded="true"] { background: var(--accent); color: var(--surface); border-color: var(--accent); }
.ex .reveal { margin-top: 12px; padding-top: 10px; border-top: 1px dashed var(--line); }
.ex .reveal h4 { margin: 0 0 4px; font-size: .78rem; letter-spacing: .1em; text-transform: uppercase; color: var(--muted); }
.ex .reveal ol { margin: 0; padding-left: 1.3em; }
.ex .answer { background: var(--sunk); border-radius: 10px; padding: 10px 14px; }
.ex .answer p { margin: .3em 0; }
.ex .credit { margin-top: 10px; font-size: .92rem; }
.ex .credit ul { margin: 4px 0 0; padding-left: 1.2em; }

.pager { display: flex; justify-content: space-between; gap: 12px; margin-top: 48px; padding-top: 18px; border-top: 1px solid var(--line); }
.pager a { text-decoration: none; max-width: 48%; }
.pager small { display: block; color: var(--muted); font-size: .75rem; text-transform: uppercase; letter-spacing: .08em; }
.pager .next { text-align: right; margin-left: auto; }

/* home */
.hero h1 { font-size: clamp(2.1rem, 5vw, 3rem); margin: 0 0 10px; }
.cards { display: grid; grid-template-columns: repeat(auto-fill, minmax(240px, 1fr)); gap: 14px; margin: 18px 0 30px; }
.card { display: block; text-decoration: none; color: inherit; background: var(--surface); border: 1px solid var(--line); border-radius: 14px; padding: 16px 18px; box-shadow: var(--shadow); transition: border-color .15s, transform .15s; }
.card:hover { border-color: var(--accent); transform: translateY(-2px); }
.card .n { font-size: .74rem; letter-spacing: .1em; text-transform: uppercase; font-weight: 600; color: var(--accent); }
.card h3 { margin: 2px 0 6px; font-size: 1.15rem; }
.card p { margin: 0; color: var(--muted); font-size: .9rem; }
.bar { height: 5px; border-radius: 9px; background: var(--sunk); margin-top: 12px; overflow: hidden; }
.bar i { display: block; height: 100%; background: var(--good); border-radius: 9px; }
.howto { display: grid; grid-template-columns: repeat(auto-fit, minmax(180px, 1fr)); gap: 10px; margin: 12px 0 26px; padding: 0; list-style: none; }
.howto li { background: var(--surface); border: 1px solid var(--line); border-radius: 12px; padding: 12px 14px; font-size: .92rem; }
.howto b { display: block; }
.topic-list { list-style: none; padding: 0; margin: 12px 0; display: grid; gap: 8px; }
.topic-list a { display: flex; gap: 12px; align-items: baseline; text-decoration: none; color: var(--ink); background: var(--surface); border: 1px solid var(--line); border-radius: 12px; padding: 12px 16px; }
.topic-list a:hover { border-color: var(--accent); }
.topic-list .ix { color: var(--muted); font-variant-numeric: tabular-nums; font-size: .85rem; }
.topic-list span.d { color: var(--muted); font-size: .88rem; display: block; }
.topic-list .ok { margin-left: auto; color: var(--good); }

/* figures */
figure.fig { margin: 12px 0; }
figure.fig svg { display: block; width: 100%; max-width: 360px; height: auto; background: var(--surface); border: 1px solid var(--line); border-radius: 12px; }
figure.fig.wide svg { max-width: 560px; }
figure.fig figcaption { color: var(--muted); font-size: .85rem; margin-top: 4px; max-width: 560px; }
.fig .grid { stroke: var(--grid); stroke-width: 1; }
.fig .axis { stroke: var(--axis); stroke-width: 1.4; }
.fig .tick { fill: var(--muted); font: 11px Inter, sans-serif; }
.fig .lbl { font: 600 12px Inter, sans-serif; }
.fig .k1 { stroke: var(--c1); fill: var(--c1); } .fig .k2 { stroke: var(--c2); fill: var(--c2); } .fig .k3 { stroke: var(--c3); fill: var(--c3); } .fig .k0 { stroke: var(--ink); fill: var(--ink); }
.fig path.curve { fill: none; stroke-width: 2.4; }
.fig .shade { stroke: none; opacity: .14; }
.fig circle.open { fill: var(--surface); stroke-width: 2; }
.fig text.k1, .fig text.k2, .fig text.k3, .fig text.k0 { stroke: var(--surface); stroke-width: 4px; stroke-linejoin: round; paint-order: stroke; }

@media (max-width: 900px) {
  .layout { grid-template-columns: minmax(0, 1fr); }
  .menu { display: inline-grid; place-items: center; }
  .nav { position: fixed; top: 56px; left: 0; bottom: 0; width: min(330px, 88vw); height: auto; z-index: 30; background: var(--bg); transform: translateX(-102%); transition: transform .2s ease; box-shadow: var(--shadow); }
  body.nav-open .nav { transform: none; }
  .main { padding: 22px 16px 64px; }
  .top { padding: 0 12px; gap: 10px; }
  .top .home { display: none; }
  .work td.why { width: auto; }
  .work tr { display: grid; }
  .work td { border-bottom: 0; }
  .work tr { border-bottom: 1px dashed var(--line); padding: 4px 0; }
}
@media print {
  .top, .nav, .jump, .btns, .pager, .done-toggle { display: none !important; }
  .layout { display: block; }
  .main { padding: 0; }
  .reveal[hidden] { display: block !important; }
  .ex, .err, .work { box-shadow: none; break-inside: avoid; }
}