/* Derivations First — shared stylesheet
   Aesthetic: engineering graph-paper. Deliberately avoids the cream+terracotta
   AI-design tell. Blueprint blue, ink, rust reserved for warnings/traps. */

:root{
  --paper:#F5F7F6; --ink:#1C2733; --ink-soft:#46535F; --rule:#C9D2D6;
  --blue:#2B5DA8; --blue-dk:#1E4176; --rust:#A64B2A; --rust-dk:#7E3418;
  --grid:rgba(43,93,168,.07); --box:#FFFFFF; --box-alt:#EEF2F1;
}
*{margin:0;padding:0;box-sizing:border-box}
html{scroll-behavior:smooth}
body{background:var(--paper);color:var(--ink);
  font-family:'Source Serif 4',Georgia,serif;font-size:17px;line-height:1.66;
  -webkit-font-smoothing:antialiased}
.page{max-width:760px;margin:0 auto;padding:0 28px 110px}
a{color:var(--blue);text-decoration:none}
a:hover{text-decoration:underline}
a:focus-visible{outline:2px solid var(--blue);outline-offset:2px}
em{font-style:italic}
strong{font-weight:600}

/* ---- running header / chapter nav ---- */
.chrome{position:sticky;top:0;z-index:20;background:rgba(245,247,246,.92);
  backdrop-filter:blur(6px);border-bottom:1px solid var(--rule)}
.chrome-in{max-width:760px;margin:0 auto;padding:10px 28px;display:flex;
  justify-content:space-between;align-items:center;gap:16px}
.chrome a{font-family:'IBM Plex Mono',monospace;font-size:12px;letter-spacing:.04em}
.chrome .home{font-weight:600;color:var(--ink)}
.chrome .nav{display:flex;gap:18px}

/* ---- cover (index only) ---- */
.cover{padding:88px 0 56px;border-bottom:3px solid var(--ink);
  background:linear-gradient(var(--grid) 1px,transparent 1px),
   linear-gradient(90deg,var(--grid) 1px,transparent 1px);background-size:28px 28px}
.imprint{font-family:'IBM Plex Mono',monospace;font-size:12px;letter-spacing:.18em;
  text-transform:uppercase;color:var(--blue-dk)}
h1.title{font-family:'Zilla Slab',serif;font-weight:700;font-size:clamp(42px,7vw,68px);
  line-height:1.02;margin:18px 0 12px}
.subtitle{font-family:'Zilla Slab',serif;font-weight:500;font-size:clamp(18px,3vw,23px);
  color:var(--ink-soft);max-width:36ch;line-height:1.3}
.cover-meta{margin-top:34px;display:flex;gap:26px;flex-wrap:wrap;
  font-family:'IBM Plex Mono',monospace;font-size:12.5px;color:var(--ink-soft)}
.cover-meta b{color:var(--ink);font-weight:600}

/* ---- chapter opener ---- */
.opener{padding:52px 0 26px;margin-bottom:8px;border-bottom:2px solid var(--ink);
  background:linear-gradient(var(--grid) 1px,transparent 1px),
   linear-gradient(90deg,var(--grid) 1px,transparent 1px);background-size:28px 28px}
.eyebrow{font-family:'IBM Plex Mono',monospace;font-size:12px;letter-spacing:.16em;
  text-transform:uppercase;color:var(--blue-dk)}
h1.ch{font-family:'Zilla Slab',serif;font-weight:700;font-size:clamp(32px,5.5vw,50px);
  line-height:1.04;margin:14px 0 14px}
.ch-blurb{font-family:'Source Serif 4',serif;font-size:19px;color:var(--ink-soft);
  max-width:60ch;font-style:italic}

/* ---- headings ---- */
h2{font-family:'Zilla Slab',serif;font-weight:600;font-size:27px;margin:58px 0 14px;
  padding-top:16px;border-top:2px solid var(--ink)}
h3{font-family:'Zilla Slab',serif;font-weight:600;font-size:20px;margin:32px 0 8px;color:var(--blue-dk)}
h4{font-family:'IBM Plex Mono',monospace;font-weight:600;font-size:13px;letter-spacing:.08em;
  text-transform:uppercase;color:var(--ink-soft);margin:24px 0 6px}

/* ---- body ---- */
p{margin:0 0 16px;max-width:68ch}
p.lead{font-size:19px}
ul,ol{margin:0 0 16px 26px}
li{margin-bottom:8px;max-width:66ch}
hr{border:none;border-top:1px solid var(--rule);margin:34px 0}

/* ---- derivation / worked-example / trap boxes ---- */
.box{background:var(--box);border:1px solid var(--rule);border-radius:2px;
  padding:20px 22px;margin:24px 0;box-shadow:0 1px 0 rgba(28,39,51,.04)}
.box .label{font-family:'IBM Plex Mono',monospace;font-size:11px;letter-spacing:.14em;
  text-transform:uppercase;display:block;margin-bottom:10px;font-weight:600}
.deriv{border-left:4px solid var(--blue);
  background:linear-gradient(var(--grid) 1px,transparent 1px),
   linear-gradient(90deg,var(--grid) 1px,transparent 1px);background-size:22px 22px;background-color:#fff}
.deriv .label{color:var(--blue-dk)}
.worked{border-left:4px solid var(--ink)}
.worked .label{color:var(--ink)}
.trap{border-left:4px solid var(--rust);background:#FBF3EF}
.trap .label{color:var(--rust-dk)}
.box p:last-child,.box ul:last-child,.box ol:last-child{margin-bottom:0}
.box .title{font-family:'Zilla Slab',serif;font-weight:600;font-size:17px;color:var(--ink);margin-bottom:6px}

/* ---- inline drill ---- */
.drill{background:var(--box-alt);border-radius:2px;padding:16px 20px;margin:22px 0}
.drill .label{font-family:'IBM Plex Mono',monospace;font-size:11px;letter-spacing:.14em;
  text-transform:uppercase;color:var(--blue-dk);font-weight:600;display:block;margin-bottom:8px}
details{margin-top:10px;border-top:1px dashed var(--rule);padding-top:8px}
summary{font-family:'IBM Plex Mono',monospace;font-size:12px;color:var(--blue);cursor:pointer;
  letter-spacing:.04em}
summary:hover{text-decoration:underline}
details[open] summary{margin-bottom:8px}

/* ---- exercises ---- */
.exset{margin-top:14px}
.exgrade{font-family:'IBM Plex Mono',monospace;font-size:12px;letter-spacing:.1em;
  text-transform:uppercase;color:var(--rust-dk);font-weight:600;margin:22px 0 4px}
ol.ex{margin-left:24px}
ol.ex li{margin-bottom:12px}

/* ---- reading list ---- */
.readings{background:var(--box);border:1px solid var(--rule);padding:18px 22px;margin:22px 0}
.readings h4{margin-top:0}
.readings ul{list-style:none;margin-left:0}
.readings li{padding:7px 0;border-bottom:1px solid var(--rule);max-width:none}
.readings li:last-child{border-bottom:none}
.readings .how{display:block;font-size:14.5px;color:var(--ink-soft);margin-top:2px}

/* ---- key-number callout ---- */
.keynum{font-family:'IBM Plex Mono',monospace;font-weight:600;color:var(--blue-dk)}

/* ---- gate banner ---- */
.gate{border:2px solid var(--ink);border-radius:2px;padding:18px 22px;margin:34px 0;
  background:var(--box)}
.gate .label{font-family:'IBM Plex Mono',monospace;font-size:11px;letter-spacing:.16em;
  text-transform:uppercase;color:var(--ink);font-weight:600;display:block;margin-bottom:8px}

/* ---- footer ---- */
footer{margin-top:80px;padding-top:18px;border-top:1px solid var(--rule);
  font-family:'IBM Plex Mono',monospace;font-size:12px;color:var(--ink-soft);
  display:flex;justify-content:space-between;flex-wrap:wrap;gap:12px}

/* ---- KaTeX sizing ---- */
.katex{font-size:1.02em}
.katex-display{margin:18px 0;overflow-x:auto;overflow-y:hidden;padding:2px 0}

@media (prefers-reduced-motion:reduce){html{scroll-behavior:auto}}
@media (max-width:640px){
  body{font-size:16px}
  .page{padding:0 20px 80px}
  .chrome-in{padding:10px 20px}
  h1.ch{font-size:32px}
}


/* ---- cover figure (index) ---- */
.cover{padding:44px 0 40px;background:none;border-bottom:3px solid var(--ink)}
.cover-fig{width:100%;margin-bottom:8px}
.coverfig{width:100%;height:auto;display:block}
.cover-cap{font-family:'IBM Plex Mono',monospace;font-size:10.5px;letter-spacing:.08em;
  color:#8A97A0;text-align:right;margin-bottom:30px}
.cover-rule{border:none;border-top:2px solid var(--ink);margin:0 0 24px}
h1.title{font-size:clamp(38px,7vw,62px);margin:0 0 18px}
.cover-author{margin-top:32px;padding-top:16px;border-top:1px solid var(--rule);
  font-family:'IBM Plex Mono',monospace;font-size:13px;letter-spacing:.14em;
  text-transform:uppercase;color:var(--ink)}
.pdf-link{display:inline-block;margin-top:20px;padding:10px 20px;
  font-family:'IBM Plex Mono',monospace;font-size:13px;letter-spacing:.06em;
  color:var(--blue);border:1.5px solid var(--blue);border-radius:3px}
.pdf-link:hover{background:var(--blue);color:#fff;text-decoration:none}