/* ============================================================
   TEXTBOOK LAYOUT — used by the homepage (Algebra).
   Loads after style.css and reuses its colors and nav.
   ============================================================ */
:root{--nav:62px;--subj:52px;--stick:114px;--side:300px;--paper:#fff;--rule:#2546e8;--rule-bg:#f3f5fe;--ex-bg:#fbfaf6;--warn-bg:#fff7ec;--check-bg:#f4f9f6;--check:#1f8a5b}
html{scroll-padding-top:calc(var(--stick) + 18px)}
body.tb{overflow-x:clip}
html:has(body.tb){overflow-x:clip}
body.tb nav{height:var(--nav);padding-top:0;padding-bottom:0}

/* ---------- subject tabs ---------- */
.subjects{position:sticky;top:var(--nav);z-index:35;height:var(--subj);display:flex;align-items:flex-end;gap:6px;padding:0 42px;
  background:rgba(247,247,244,.94);backdrop-filter:blur(10px);border-bottom:1px solid var(--line)}
.subj{display:flex;align-items:center;gap:10px;height:42px;padding:0 16px 0 7px;border:1px solid var(--line);border-bottom:none;border-radius:9px 9px 0 0;
  font-weight:600;font-size:14.5px;color:var(--muted);background:transparent;position:relative;top:1px;transition:.18s}
.subj:hover{color:var(--ink);background:#fff}
.subj.on{background:#fff;color:var(--ink);border-color:var(--line)}
.subj.on::after{content:'';position:absolute;left:0;right:0;top:-1px;height:2px;background:var(--cobalt);border-radius:9px 9px 0 0}
.subj-ico{display:grid;place-items:center;min-width:38px;height:28px;padding:0 6px;border-radius:6px;background:var(--cobalt);color:#fff;
  font-family:'Space Mono',monospace;font-size:12.5px;font-weight:700;letter-spacing:-.02em}
.subj:not(.on) .subj-ico{background:#e9ecf6;color:var(--cobalt)}

.progress{position:sticky;top:var(--stick);z-index:34;height:2px;background:transparent}
.progress span{display:block;height:100%;width:0;background:var(--amber)}

/* ---------- two-column book ---------- */
.book-wrap{display:grid;grid-template-columns:var(--side) minmax(0,1fr);max-width:1320px;margin:0 auto}
.toc{position:sticky;top:calc(var(--stick) + 2px);height:calc(100vh - var(--stick) - 2px);overflow-y:auto;border-right:1px solid var(--line);
  padding:26px 18px 60px 34px;background:rgba(247,247,244,.7);scrollbar-width:thin}
.toc-head{display:flex;justify-content:space-between;align-items:center;font-family:'Space Mono',monospace;font-size:11px;letter-spacing:.2em;
  text-transform:uppercase;color:var(--muted);margin-bottom:16px}
.toc-x{display:none}
.toc-list,.toc-list ol{list-style:none}
.toc-ch{margin-bottom:12px}
.toc a{display:flex;gap:10px;padding:5px 10px;border-radius:6px;color:var(--muted);font-size:13.5px;line-height:1.35;transition:.15s;border-left:2px solid transparent}
.toc a:hover{color:var(--ink);background:#fff}
.toc a .n{font-family:'Space Mono',monospace;font-size:11.5px;color:var(--muted);min-width:22px;padding-top:1px}
.toc a.ch{color:var(--ink);font-weight:600;font-size:14px}
.toc a.ch .n{color:var(--cobalt);font-weight:700}
.toc ol ol a{padding-left:20px;font-size:13px}
.toc ol ol a .n{min-width:28px}
.toc a.on{color:var(--cobalt);background:#fff;border-left-color:var(--cobalt)}
.toc a.on .n{color:var(--cobalt)}
.toc a.ch.in{color:var(--cobalt)}

/* ---------- reading column ---------- */
.book{max-width:820px;width:100%;padding:56px 64px 90px;font-family:'Source Serif 4',Georgia,serif;font-size:17.5px;line-height:1.7;color:#22211c}
.book p{margin:0 0 16px}
.book b{font-weight:600;color:var(--ink)}
.book h2,.book h3,.book .box-h,.book .tp-k,.book .ch-k{font-family:'Space Grotesk',sans-serif}

.title-page{padding-bottom:40px;margin-bottom:30px;border-bottom:2px solid var(--ink)}
.tp-k,.ch-k{font-family:'Space Mono',monospace!important;font-size:12px;letter-spacing:.2em;text-transform:uppercase;color:var(--cobalt);margin-bottom:14px}
.title-page h1{font-family:'Space Grotesk',sans-serif;font-size:clamp(52px,8vw,92px);font-weight:700;letter-spacing:-.045em;line-height:.95;color:var(--ink)}
.tp-sub{font-family:'Space Grotesk',sans-serif;font-size:21px;color:var(--muted);margin:14px 0 30px!important}
.legend{display:grid;grid-template-columns:1fr 1fr;gap:10px;margin-top:26px;font-size:14.5px;line-height:1.45}
.lg{background:#fff;border:1px solid var(--line);border-left:3px solid;border-radius:8px;padding:11px 14px;color:var(--muted)}
.lg b{display:block;font-family:'Space Grotesk',sans-serif;color:var(--ink);font-size:14px}
.lg.rule{border-left-color:var(--rule)}.lg.ex{border-left-color:var(--steel)}.lg.warn{border-left-color:var(--amber)}.lg.check{border-left-color:var(--check)}

.chapter{padding-top:34px}
.ch-head{padding:30px 0 26px;margin-bottom:10px;border-bottom:1px solid var(--line)}
.ch-head h2{font-size:clamp(30px,4.2vw,44px);font-weight:700;letter-spacing:-.03em;line-height:1.05;color:var(--ink);margin-bottom:12px}
.ch-head>p{color:var(--muted);font-size:18px}
.ch-mini{list-style:none;display:flex;flex-wrap:wrap;gap:6px 8px;margin-top:16px}
.ch-mini a{font-family:'Space Mono',monospace;font-size:11.5px;color:var(--muted);border:1px solid var(--line);background:#fff;padding:4px 9px;border-radius:5px;transition:.15s}
.ch-mini a:hover{color:var(--cobalt);border-color:var(--cobalt)}

.sec{padding-top:26px}
.sec h3{display:flex;align-items:baseline;gap:12px;font-size:24px;font-weight:700;letter-spacing:-.02em;color:var(--ink);margin:8px 0 16px}
.sec h3 .n{font-family:'Space Mono',monospace;font-size:15px;color:var(--cobalt);font-weight:700}

/* ---------- boxes ---------- */
.box{border:1px solid var(--line);border-left:3px solid var(--line);border-radius:10px;margin:22px 0;background:var(--paper);overflow:hidden}
.box-h{display:flex;align-items:baseline;gap:10px;flex-wrap:wrap;padding:12px 18px 0}
.box-k{font-family:'Space Mono',monospace;font-size:11px;letter-spacing:.14em;text-transform:uppercase;font-weight:700}
.box-n{font-weight:600;font-size:15.5px;color:var(--ink)}
.box-b{padding:6px 18px 4px}
.box-b p:last-child{margin-bottom:12px}
.rule,.def{border-left-color:var(--rule);background:var(--rule-bg)}
.rule .box-k,.def .box-k{color:var(--rule)}
.def{background:#fff}
.ex{border-left-color:var(--steel);background:var(--ex-bg)}.ex .box-k{color:var(--steel)}
.warn{border-left-color:var(--amber);background:var(--warn-bg)}.warn .box-k{color:#b86400}
.note{border-left-color:#b9b6aa}.note .box-k{color:var(--muted)}
.check{border-left-color:var(--check);background:var(--check-bg)}.check .box-k{color:var(--check)}

.eq{font-family:'STIX Two Text',serif;font-size:24px;text-align:center;padding:16px 18px 14px;color:var(--ink);line-height:1.5;overflow-x:auto}
.rule .eq,.def .eq{border-bottom:1px dashed rgba(37,70,232,.2);margin:0 18px}
.eq.display{margin:18px 0;background:#fff;border:1px solid var(--line);border-radius:10px}
.box-b .eq-line{font-family:'STIX Two Text',serif;text-align:center;font-size:19px}

ol.work{list-style:none;counter-reset:w;margin:0 0 12px;padding:0}
ol.work li{counter-increment:w;position:relative;padding:4px 0 4px 34px}
ol.work li::before{content:counter(w);position:absolute;left:0;top:7px;width:21px;height:21px;border-radius:50%;border:1px solid var(--line);background:#fff;
  font-family:'Space Mono',monospace;font-size:11px;display:grid;place-items:center;color:var(--steel)}
ol.steps{margin:6px 0 18px 24px}ol.steps li{margin-bottom:8px;padding-left:4px}

ol.qs{list-style:none;counter-reset:q;padding:8px 18px 14px;display:grid;gap:10px}
ol.qs li{counter-increment:q;background:#fff;border:1px solid var(--line);border-radius:8px;padding:10px 14px 10px 44px;position:relative}
ol.qs li::before{content:counter(q);position:absolute;left:14px;top:11px;font-family:'Space Mono',monospace;font-size:12px;font-weight:700;color:var(--check)}
details summary{cursor:pointer;font-family:'Space Mono',monospace;font-size:11.5px;letter-spacing:.06em;color:var(--check);margin-top:4px;list-style:none;width:max-content}
details summary::-webkit-details-marker{display:none}
details summary::before{content:'+ '}details[open] summary::before{content:'− '}
details[open] summary{margin-bottom:4px}
.ans{border-top:1px dashed var(--line);padding-top:6px;color:var(--ink)}

/* ---------- inline math ---------- */
.m{font-family:'STIX Two Text',serif;font-size:1.06em;white-space:nowrap}
.m i,.eq i{font-style:italic}
.tx{font-style:normal;font-family:inherit}
.eq .tx{font-family:'Source Serif 4',serif;font-size:.82em;white-space:normal}
.q{display:inline-block;width:.6em}.qq{display:inline-block;width:1.6em}.th{display:inline-block;width:.25em}
.fr{display:inline-flex;flex-direction:column;vertical-align:middle;text-align:center;font-size:.9em;line-height:1.15;margin:0 .1em}
.fr>span:first-child{border-bottom:1px solid currentColor;padding:0 .15em}
.fr>span:last-child{padding:0 .15em}
.arr{display:inline-flex;flex-direction:column;gap:4px;text-align:center}
sup{font-size:.7em;line-height:0}

/* ---------- figures ---------- */
.fig{margin:24px 0}
.fig svg{width:100%;height:auto;display:block;background:#fff;border:1px solid var(--line);border-radius:10px}
.fig figcaption{font-family:'Space Mono',monospace;font-size:12px;color:var(--muted);margin-top:8px;line-height:1.5}
.fig .ax,.fig .tk{stroke:var(--ink);stroke-width:1.5}.fig .ah{fill:var(--ink)}
.fig .lb{font-family:'STIX Two Text',serif;font-size:17px;text-anchor:middle;fill:var(--ink)}
.fig .lb.neg{fill:var(--amber)}.fig .lb.zero{font-weight:700}
.fig .cap{font-family:'Space Mono',monospace;font-size:11px;text-anchor:middle;fill:var(--cobalt);letter-spacing:.1em}.fig .cap.neg{fill:var(--amber)}
.fig .ra{fill:#e8edfd;stroke:var(--cobalt);stroke-width:1.5}.fig .rb{fill:#fdf0df;stroke:var(--amber);stroke-width:1.5}
.fig .big{font-family:'STIX Two Text',serif;font-style:italic;font-size:30px;text-anchor:middle;fill:var(--ink)}
.fig .dim{font-family:'STIX Two Text',serif;font-style:italic;font-size:19px;text-anchor:middle;fill:var(--muted)}
.fig .brace{stroke:var(--muted);fill:none;stroke-width:1.2}

.book-end{margin-top:60px;padding:30px;border:1px solid var(--line);border-radius:12px;background:#fff;font-family:'Space Grotesk',sans-serif}
.book-end p{color:var(--muted);margin:6px 0 18px!important}

/* ---------- mobile drawer ---------- */
.toc-fab{display:none}
.scrim{position:fixed;inset:0;background:rgba(20,19,15,.35);z-index:60}
@media(max-width:960px){
  .book-wrap{grid-template-columns:1fr}
  .toc{position:fixed;top:0;left:0;bottom:0;height:auto;width:min(330px,86vw);z-index:70;background:var(--bg);transform:translateX(-102%);transition:transform .25s;box-shadow:8px 0 30px rgba(0,0,0,.12);padding:20px 16px 60px 20px}
  .toc.open{transform:none}
  .toc-x{display:block;background:none;border:1px solid var(--line);border-radius:6px;width:32px;height:32px;cursor:pointer;color:var(--muted)}
  .toc-fab{display:block;position:fixed;left:16px;bottom:16px;z-index:50;font-family:'Space Mono',monospace;font-size:12.5px;letter-spacing:.04em;
    background:var(--ink);color:var(--bg);border:none;border-radius:999px;padding:12px 18px;cursor:pointer;box-shadow:0 8px 24px rgba(0,0,0,.2)}
  .book{padding:40px 36px 90px;margin:0 auto}
}
@media(max-width:560px){
  :root{--subj:48px}
  .subjects{padding:0 16px}
  .book{padding:30px 16px 90px;font-size:16.5px}
  .legend{grid-template-columns:1fr}
  .eq{font-size:20px;padding:14px 10px 12px}
  .box-h{padding:11px 14px 0}.box-b{padding:6px 14px 4px}ol.qs{padding:8px 12px 12px}
  .rule .eq,.def .eq{margin:0 10px}
  .m{white-space:normal}
}
.prob{font-weight:600}
ul.cases{list-style:none;margin:2px 0 12px;padding:0}
ul.cases li{position:relative;padding:3px 0 3px 22px}
ul.cases li::before{content:'';position:absolute;left:4px;top:15px;width:6px;height:6px;border-radius:50%;background:var(--steel);opacity:.5}