:root {
  --paper: #f4efe4;
  --paper-2: #efe8d8;
  --ink: #20211d;
  --ink-soft: #5a5a4f;
  --rule: #cfc6b0;
  --accent: #7a1f1f;       /* oxblood */
  --accent-2: #1f4d4a;     /* deep teal */
  --gold: #b08428;
  --ok: #2f6b3a;
  --shadow: 0 1px 0 rgba(0,0,0,.04), 0 18px 40px -28px rgba(40,30,10,.55);
  --maxw: 880px;
}

* { box-sizing: border-box; }
html { scroll-behavior: smooth; }

body {
  margin: 0;
  background:
    radial-gradient(1200px 600px at 80% -10%, rgba(176,132,40,.10), transparent 60%),
    radial-gradient(900px 500px at -10% 10%, rgba(31,77,74,.08), transparent 55%),
    var(--paper);
  color: var(--ink);
  font-family: "Newsreader", Georgia, serif;
  font-size: 18px;
  line-height: 1.55;
  background-attachment: fixed;
}
body::before {
  content: "";
  position: fixed;
  inset: 0;
  pointer-events: none;
  opacity: .035;
  z-index: 0;
  background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='120' height='120'%3E%3Cfilter id='n'%3E%3CfeTurbulence type='fractalNoise' baseFrequency='0.9' numOctaves='2'/%3E%3C/filter%3E%3Crect width='100%25' height='100%25' filter='url(%23n)'/%3E%3C/svg%3E");
}

/* ---------- sticky exam header ---------- */
header.bar {
  position: sticky;
  top: 0;
  z-index: 50;
  background: rgba(244,239,228,.86);
  backdrop-filter: blur(8px);
  border-bottom: 1px solid var(--rule);
}
.bar-inner {
  max-width: var(--maxw);
  margin: 0 auto;
  padding: 10px 22px;
  display: flex;
  align-items: center;
  gap: 16px;
  justify-content: space-between;
}
.brand {
  font-family: "Fraunces", serif;
  font-weight: 800;
  letter-spacing: .01em;
  font-size: 17px;
  color: var(--accent);
  display: flex;
  align-items: baseline;
  gap: 10px;
  text-decoration: none;
}
.brand small {
  font-family: "JetBrains Mono", monospace;
  font-weight: 400;
  font-size: 11px;
  letter-spacing: .12em;
  text-transform: uppercase;
  color: var(--ink-soft);
}
.timer { display: flex; align-items: center; gap: 12px; }
.clock {
  font-family: "JetBrains Mono", monospace;
  font-size: 18px;
  font-weight: 500;
  padding: 4px 12px;
  border: 1px solid var(--rule);
  border-radius: 999px;
  background: var(--paper-2);
  min-width: 92px;
  text-align: center;
}
.clock.warn { color: var(--accent); border-color: var(--accent); }
.btn {
  font-family: "JetBrains Mono", monospace;
  font-size: 12px;
  letter-spacing: .04em;
  text-transform: uppercase;
  cursor: pointer;
  border: 1px solid var(--ink);
  background: var(--ink);
  color: var(--paper);
  padding: 7px 12px;
  border-radius: 7px;
  transition: transform .08s ease, opacity .15s ease;
  text-decoration: none;
  display: inline-block;
}
.btn:hover { transform: translateY(-1px); }
.btn:active { transform: translateY(0); }
.btn.ghost { background: transparent; color: var(--ink); }
.btn.small { padding: 5px 10px; font-size: 11px; }

.progress { height: 3px; background: var(--rule); position: relative; }
.progress > span {
  position: absolute;
  left: 0; top: 0; bottom: 0;
  width: 0%;
  background: linear-gradient(90deg, var(--accent), var(--gold));
  transition: width .3s ease;
}

/* ---------- layout ---------- */
main {
  max-width: var(--maxw);
  margin: 0 auto;
  padding: 0 22px 140px;
  position: relative;
  z-index: 1;
}

.cover {
  text-align: center;
  padding: 64px 0 30px;
  border-bottom: 2px solid var(--ink);
  margin-bottom: 8px;
}
.cover .kicker {
  font-family: "JetBrains Mono", monospace;
  font-size: 12px;
  letter-spacing: .3em;
  text-transform: uppercase;
  color: var(--accent-2);
}
.cover h1 {
  font-family: "Fraunces", serif;
  font-weight: 800;
  font-size: clamp(34px, 6vw, 58px);
  line-height: 1.02;
  margin: 14px 0 6px;
  letter-spacing: -.01em;
}
.cover .sub { font-style: italic; color: var(--ink-soft); font-size: 19px; }
.meta {
  display: flex;
  flex-wrap: wrap;
  justify-content: center;
  gap: 8px 22px;
  margin-top: 22px;
  font-family: "JetBrains Mono", monospace;
  font-size: 12.5px;
  color: var(--ink-soft);
  letter-spacing: .03em;
}
.meta b { color: var(--ink); font-weight: 500; }

.instructions {
  background: var(--paper-2);
  border: 1px solid var(--rule);
  border-left: 4px solid var(--accent);
  border-radius: 6px;
  padding: 16px 20px;
  margin: 26px 0;
  font-size: 16px;
}
.instructions h3 {
  margin: 0 0 8px;
  font-family: "Fraunces", serif;
  font-size: 17px;
  letter-spacing: .02em;
  text-transform: uppercase;
}
.instructions ul { margin: 0; padding-left: 20px; }
.instructions li { margin: 3px 0; }

/* constants box */
details.constants {
  border: 1px dashed var(--accent-2);
  border-radius: 6px;
  margin: 22px 0 8px;
  background: rgba(31,77,74,.05);
}
details.constants > summary {
  cursor: pointer;
  padding: 12px 18px;
  font-family: "JetBrains Mono", monospace;
  font-size: 12px;
  letter-spacing: .12em;
  text-transform: uppercase;
  color: var(--accent-2);
  list-style: none;
}
details.constants > summary::-webkit-details-marker { display: none; }
details.constants > summary::before { content: "\25B8  "; }
details.constants[open] > summary::before { content: "\25BE  "; }
.const-grid {
  padding: 4px 20px 18px;
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(190px, 1fr));
  gap: 6px 24px;
  font-size: 15px;
}
.const-grid div { border-bottom: 1px dotted var(--rule); padding: 3px 0; }

/* ---------- parts ---------- */
.part { margin-top: 54px; scroll-margin-top: 80px; }
.part-head {
  display: flex;
  align-items: flex-end;
  justify-content: space-between;
  gap: 16px;
  border-bottom: 2px solid var(--ink);
  padding-bottom: 8px;
}
.part-head .num {
  font-family: "JetBrains Mono", monospace;
  font-size: 12px;
  letter-spacing: .25em;
  text-transform: uppercase;
  color: var(--accent);
}
.part-head h2 {
  font-family: "Fraunces", serif;
  font-weight: 700;
  font-size: clamp(26px, 4.5vw, 38px);
  margin: 2px 0 0;
  letter-spacing: -.01em;
}
.part-head .pace { font-style: italic; color: var(--ink-soft); font-size: 15px; white-space: nowrap; }
.section-label {
  font-family: "JetBrains Mono", monospace;
  font-size: 11.5px;
  letter-spacing: .18em;
  text-transform: uppercase;
  color: var(--accent-2);
  margin: 26px 0 4px;
}

/* ---------- question card ---------- */
.q {
  margin: 18px 0;
  padding: 18px 20px 16px;
  background: rgba(255,255,255,.45);
  border: 1px solid var(--rule);
  border-radius: 10px;
  box-shadow: var(--shadow);
}
.q-top { display: flex; gap: 12px; align-items: baseline; }
.q-id {
  font-family: "JetBrains Mono", monospace;
  font-weight: 500;
  font-size: 14px;
  color: var(--accent);
  flex-shrink: 0;
  padding-top: 1px;
}
.q-prompt { flex: 1; }
.q-prompt p { margin: 0 0 8px; }
.q-prompt p:last-child { margin-bottom: 0; }
.q-prompt ul, .q-prompt ol { margin: 6px 0; }

.pill {
  font-family: "JetBrains Mono", monospace;
  font-size: 10px;
  letter-spacing: .08em;
  text-transform: uppercase;
  padding: 2px 8px;
  border-radius: 999px;
  border: 1px solid var(--rule);
  color: var(--ink-soft);
  white-space: nowrap;
}
.pill.concept { color: var(--accent-2); border-color: var(--accent-2); }
.pill.numeric { color: var(--gold); border-color: var(--gold); }

/* ---------- figures ---------- */
.figure {
  margin: 14px 0 4px;
  padding: 14px 14px 8px;
  background: #fffdf7;
  border: 1px solid var(--rule);
  border-radius: 8px;
  text-align: center;
}
.figure svg { max-width: 100%; height: auto; color: var(--ink); }
.figure .cap {
  margin-top: 8px;
  font-size: 13px;
  font-style: italic;
  color: var(--ink-soft);
}
.figure .cap b { font-style: normal; font-family: "JetBrains Mono", monospace; font-weight: 500; color: var(--accent); letter-spacing: .04em; }
.svg-fill-sub { fill: #e7decb; }
.svg-fill-nplus { fill: rgba(31,77,74,.32); }
.svg-fill-nlight { fill: rgba(31,77,74,.14); }
.svg-fill-pplus { fill: rgba(122,31,31,.30); }
.svg-fill-ox { fill: rgba(176,132,40,.30); }
.svg-fill-metal { fill: rgba(40,40,45,.55); }
.svg-gate { fill: var(--ink); }
.svg-line { fill: none; stroke: var(--ink); stroke-width: 1.6; }
.svg-curve { fill: none; stroke: var(--accent); stroke-width: 2.2; }
.svg-curve-2 { fill: none; stroke: var(--accent-2); stroke-width: 2.2; }
.svg-dash { fill: none; stroke: var(--ink-soft); stroke-width: 1.2; stroke-dasharray: 5 4; }
.svg-axis { fill: none; stroke: var(--ink); stroke-width: 1.4; }
.svg-txt { font-family: "JetBrains Mono", monospace; font-size: 12px; fill: var(--ink); }
.svg-txt.sm { font-size: 10.5px; fill: var(--ink-soft); }
.svg-txt.w { fill: var(--paper); }
.svg-txt.acc { fill: var(--accent); }

/* answer area */
.answer { margin-top: 12px; }
.answer textarea {
  width: 100%;
  min-height: 84px;
  resize: vertical;
  font-family: "JetBrains Mono", monospace;
  font-size: 14px;
  line-height: 1.5;
  color: var(--ink);
  background: #fffdf7;
  border: 1px solid var(--rule);
  border-radius: 7px;
  padding: 10px 12px;
}
.answer textarea:focus { outline: none; border-color: var(--accent-2); box-shadow: 0 0 0 3px rgba(31,77,74,.12); }
.answer-tools { display: flex; flex-wrap: wrap; gap: 8px; margin-top: 8px; align-items: center; }

.latex-wrap { margin-top: 10px; display: none; }
.latex-wrap.show { display: block; }
.latex-wrap label {
  display: block;
  font-family: "JetBrains Mono", monospace;
  font-size: 10.5px;
  letter-spacing: .1em;
  text-transform: uppercase;
  color: var(--ink-soft);
  margin-bottom: 4px;
}
.latex-input {
  width: 100%;
  font-family: "JetBrains Mono", monospace;
  font-size: 14px;
  background: #fffdf7;
  border: 1px solid var(--rule);
  border-radius: 7px 7px 0 0;
  border-bottom: none;
  padding: 8px 12px;
}
.latex-input:focus { outline: none; border-color: var(--gold); }
.latex-render {
  min-height: 42px;
  border: 1px solid var(--rule);
  border-top: 1px dashed var(--rule);
  border-radius: 0 0 7px 7px;
  background: var(--paper-2);
  padding: 10px 12px;
  font-size: 17px;
  overflow-x: auto;
}
.latex-render .ph { color: var(--ink-soft); font-style: italic; font-family: "Newsreader", serif; font-size: 14px; }

/* solution */
.solution {
  display: none;
  margin-top: 14px;
  border: 1px solid var(--ok);
  border-left: 4px solid var(--ok);
  border-radius: 6px;
  background: rgba(47,107,58,.06);
  padding: 12px 16px;
}
.solution.show { display: block; }
.solution .sol-label {
  font-family: "JetBrains Mono", monospace;
  font-size: 11px;
  letter-spacing: .14em;
  text-transform: uppercase;
  color: var(--ok);
  margin-bottom: 6px;
}
.solution p { margin: 0 0 8px; }
.solution p:last-child { margin-bottom: 0; }
.solution .ans {
  font-weight: 500;
  background: rgba(176,132,40,.16);
  padding: 1px 6px;
  border-radius: 4px;
}
.solution .figure { background: rgba(255,255,255,.6); }
.katex { font-size: 1.04em; }

.disclaimer {
  margin-top: 24px;
  padding: 14px 18px;
  background: rgba(122, 31, 31, .06);
  border: 1px solid rgba(122, 31, 31, .18);
  border-radius: 8px;
  font-size: 14px;
  line-height: 1.55;
  color: var(--ink-soft);
}
.disclaimer strong {
  display: block;
  font-family: "JetBrains Mono", monospace;
  font-size: 11px;
  letter-spacing: .12em;
  text-transform: uppercase;
  color: var(--accent);
  margin-bottom: 6px;
}

footer {
  text-align: center;
  margin-top: 80px;
  padding-top: 22px;
  border-top: 2px solid var(--ink);
  font-family: "JetBrains Mono", monospace;
  font-size: 11px;
  letter-spacing: .12em;
  text-transform: uppercase;
  color: var(--ink-soft);
}

@media (max-width: 560px) {
  body { font-size: 16.5px; }
  .bar-inner { flex-wrap: wrap; gap: 8px; }
  .q-top { flex-direction: column; gap: 4px; }
}