/* ----------------------------------------------------------------------
   Inside Large Language Models — landing page
   Palette is the book's four-color system:
     amber  = computed / transformed
     blue   = input / source
     green  = output / result
     purple = aggregated
   ---------------------------------------------------------------------- */

:root {
  --amber-50:  #FAEEDA;
  --amber-200: #F8DCA9;
  --amber-500: #EF9F27;
  --amber-700: #B5751A;
  --amber-900: #633806;

  --blue-50:   #E6F1FB;
  --blue-200:  #B6D4F4;
  --blue-500:  #378ADD;
  --blue-700:  #1F5FA9;
  --blue-900:  #0C447C;

  --green-50:  #E1F5EE;
  --green-200: #A6E2CE;
  --green-500: #1D9E75;
  --green-700: #117153;
  --green-900: #085041;

  --purple-50:  #EEEDFE;
  --purple-200: #C7C4F4;
  --purple-500: #7F77DD;
  --purple-700: #5B53B3;
  --purple-900: #3C3489;

  --bg:        #FFFFFF;
  --bg-soft:   #FAF9F6;
  --bg-deep:   #0B1226;

  --ink:       #1A1A1A;
  --ink-soft:  #3A3A3A;
  --muted:     #6B7280;
  --hairline:  rgba(12, 18, 38, 0.08);

  --shadow-sm: 0 1px 2px rgba(12, 18, 38, 0.06), 0 2px 6px rgba(12, 18, 38, 0.05);
  --shadow-md: 0 4px 14px rgba(12, 18, 38, 0.07), 0 8px 30px rgba(12, 18, 38, 0.06);
  --shadow-lg: 0 12px 36px rgba(12, 18, 38, 0.10), 0 24px 60px rgba(12, 18, 38, 0.10);

  --radius-sm: 8px;
  --radius-md: 14px;
  --radius-lg: 22px;

  --font-display: "Playfair Display", "Georgia", serif;
  --font-body:    "Inter", -apple-system, BlinkMacSystemFont, "Segoe UI", system-ui, sans-serif;
  --font-mono:    "JetBrains Mono", "Menlo", "Consolas", monospace;
}

* { box-sizing: border-box; }

html { scroll-behavior: smooth; }

/* Visible-on-focus skip link for keyboard and screen-reader users. */
.skip-link {
  position: absolute;
  top: -40px;
  left: 12px;
  background: var(--blue-900);
  color: #fff !important;
  padding: 10px 16px;
  border-radius: 8px;
  font-weight: 700;
  font-size: 14px;
  z-index: 1000;
  transition: top .15s ease;
}
.skip-link:focus { top: 12px; outline: 3px solid var(--amber-500); outline-offset: 2px; }

/* Consistent visible focus across interactive elements. */
:focus-visible {
  outline: 3px solid var(--amber-500);
  outline-offset: 2px;
  border-radius: 4px;
}
.btn:focus-visible { outline-offset: 4px; }

/* Respect reduced-motion preference. */
@media (prefers-reduced-motion: reduce) {
  * { animation-duration: 0.01ms !important; transition-duration: 0.01ms !important; }
  html { scroll-behavior: auto; }
}

body {
  margin: 0;
  font-family: var(--font-body);
  font-size: 17px;
  line-height: 1.62;
  color: var(--ink);
  background: var(--bg);
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

img, svg { max-width: 100%; height: auto; display: block; }

a { color: var(--blue-700); text-decoration: none; transition: color .15s ease; }
a:hover { color: var(--blue-900); }

.container {
  width: 100%;
  max-width: 1180px;
  margin: 0 auto;
  padding: 0 24px;
}

/* ---------- Top bar ---------- */

.topbar {
  position: sticky;
  top: 0;
  z-index: 50;
  background: rgba(255, 255, 255, 0.85);
  backdrop-filter: saturate(180%) blur(12px);
  -webkit-backdrop-filter: saturate(180%) blur(12px);
  border-bottom: 1px solid var(--hairline);
}
.topbar-row {
  display: flex;
  align-items: center;
  justify-content: space-between;
  height: 64px;
}
.brand {
  display: flex;
  align-items: center;
  gap: 12px;
  font-weight: 700;
  color: var(--ink);
  letter-spacing: -0.01em;
}
.brand-mark {
  display: inline-flex;
  width: 36px;
  height: 36px;
  flex-shrink: 0;
}
.brand-mark svg {
  width: 100%;
  height: 100%;
  display: block;
  border-radius: 10px;
}
.brand-text { font-size: 15px; color: var(--ink-soft); }

.topnav {
  display: flex;
  align-items: center;
  gap: 28px;
}
.topnav a {
  font-size: 14px;
  font-weight: 600;
  color: var(--ink-soft);
}
.topnav a:hover { color: var(--blue-900); }
.nav-cta {
  background: var(--blue-900);
  color: #fff !important;
  padding: 8px 16px;
  border-radius: 999px;
  transition: background .15s ease, transform .15s ease;
}
.nav-cta:hover { background: var(--blue-700); transform: translateY(-1px); }

/* ---------- Hero ---------- */

.hero {
  position: relative;
  padding: 80px 0 88px;
  background:
    radial-gradient(900px 420px at 12% 0%, var(--amber-50), transparent 60%),
    radial-gradient(900px 420px at 90% 10%, var(--blue-50), transparent 60%),
    radial-gradient(900px 420px at 60% 100%, var(--green-50), transparent 60%),
    radial-gradient(900px 420px at 0% 100%, var(--purple-50), transparent 60%),
    var(--bg);
  overflow: hidden;
}
.hero::before {
  content: "";
  position: absolute;
  inset: 0;
  background-image:
    linear-gradient(transparent 0, transparent 39px, rgba(12, 18, 38, 0.04) 40px),
    linear-gradient(90deg, transparent 0, transparent 39px, rgba(12, 18, 38, 0.04) 40px);
  background-size: 40px 40px;
  mask-image: radial-gradient(ellipse at center, #000, transparent 70%);
  -webkit-mask-image: radial-gradient(ellipse at center, #000, transparent 70%);
  pointer-events: none;
}
.hero-grid {
  position: relative;
  display: grid;
  grid-template-columns: minmax(0, 1.25fr) minmax(0, 1fr);
  gap: 56px;
  align-items: center;
}

.kicker {
  display: inline-flex;
  align-items: center;
  gap: 10px;
  padding: 6px 14px;
  border-radius: 999px;
  background: #fff;
  border: 1px solid var(--hairline);
  box-shadow: var(--shadow-sm);
  color: var(--ink-soft);
  font-size: 13px;
  font-weight: 600;
  letter-spacing: 0.01em;
  margin-bottom: 22px;
}
.kicker .dot {
  width: 8px; height: 8px;
  border-radius: 50%;
  background: var(--green-500);
  box-shadow: 0 0 0 4px rgba(29, 158, 117, 0.18);
}

.display {
  font-family: var(--font-display);
  font-weight: 900;
  font-size: clamp(40px, 5.4vw, 68px);
  line-height: 1.05;
  letter-spacing: -0.02em;
  color: var(--ink);
  margin: 0 0 26px;
}
.hl-amber { color: var(--amber-700); }
.hl-blue  { color: var(--blue-900); }

.hero-questions {
  list-style: none;
  margin: 0 0 26px;
  padding: 0;
  display: flex;
  flex-direction: column;
  gap: 10px;
}
.hero-questions li {
  position: relative;
  padding: 14px 18px 14px 46px;
  background: #fff;
  border: 1px solid var(--hairline);
  border-radius: var(--radius-md);
  box-shadow: var(--shadow-sm);
  font-size: 16px;
  font-weight: 500;
  color: var(--ink-soft);
}
.hero-questions li::before {
  content: "?";
  position: absolute;
  left: 14px; top: 50%;
  transform: translateY(-50%);
  width: 22px; height: 22px;
  border-radius: 50%;
  display: grid;
  place-items: center;
  font-family: var(--font-display);
  font-weight: 900;
  font-size: 14px;
}
.hero-questions li:nth-child(1)::before { background: var(--amber-50);  color: var(--amber-700); }
.hero-questions li:nth-child(2)::before { background: var(--blue-50);   color: var(--blue-900); }
.hero-questions li:nth-child(3)::before { background: var(--purple-50); color: var(--purple-900); }

.hero-tag {
  font-size: 16.5px;
  color: var(--ink-soft);
  margin: 0 0 10px;
  max-width: 56ch;
}
.hero-tag.emph {
  font-weight: 700;
  color: var(--ink);
  font-size: 19px;
  margin-bottom: 26px;
}

.hero-ctas { display: flex; gap: 14px; flex-wrap: wrap; }

.btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 8px;
  padding: 13px 22px;
  border-radius: 999px;
  font-weight: 700;
  font-size: 15px;
  letter-spacing: 0.01em;
  border: 1px solid transparent;
  cursor: pointer;
  transition: transform .15s ease, box-shadow .15s ease, background .15s ease, color .15s ease;
  text-decoration: none;
}
.btn-primary {
  background: var(--blue-900);
  color: #fff !important;
  box-shadow: 0 6px 18px rgba(12, 68, 124, 0.30);
}
.btn-primary:hover {
  background: var(--blue-700);
  transform: translateY(-2px);
  box-shadow: 0 12px 24px rgba(12, 68, 124, 0.30);
}
.btn-secondary {
  background: var(--amber-500);
  color: #fff !important;
  box-shadow: 0 6px 18px rgba(239, 159, 39, 0.30);
}
.btn-secondary:hover {
  background: var(--amber-700);
  transform: translateY(-2px);
}
.btn-ghost {
  background: transparent;
  color: var(--ink) !important;
  border-color: rgba(12, 18, 38, 0.18);
}
.btn-ghost:hover {
  background: rgba(12, 18, 38, 0.04);
  transform: translateY(-2px);
}

/* ---------- Hero covers ---------- */

.hero-art {
  position: relative;
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 22px;
  align-items: end;
}
.cover {
  display: block;
  width: 100%;
  height: auto;
  border-radius: 14px;
  box-shadow:
    0 1px 0 rgba(255,255,255,0.6) inset,
    0 12px 28px rgba(12, 18, 38, 0.18),
    0 28px 70px rgba(12, 18, 38, 0.20);
  transition: transform .3s ease, box-shadow .3s ease;
  background: #fff;
}
.cover-vol1 { transform: rotate(-1.5deg); }
.cover-vol2 { transform: translateY(28px) rotate(1.5deg); }
.cover-vol1:hover { transform: rotate(-0.5deg) translateY(-6px); }
.cover-vol2:hover { transform: translateY(22px) rotate(0.5deg); }

/* ---------- Topics strip (keyword-dense, scannable) ---------- */

.topics-strip {
  padding: 36px 0 40px;
  background: #FFFFFF;
  border-top: 1px solid var(--hairline);
  border-bottom: 1px solid var(--hairline);
}
.topics-label {
  font-family: var(--font-mono);
  font-size: 12px;
  font-weight: 700;
  letter-spacing: 0.18em;
  text-transform: uppercase;
  color: var(--muted);
  margin: 0 0 18px;
  text-align: center;
}
.topics-cloud {
  list-style: none;
  margin: 0;
  padding: 0;
  display: flex;
  flex-wrap: wrap;
  justify-content: center;
  gap: 8px;
}
.topic-pill {
  font-size: 13.5px;
  font-weight: 600;
  letter-spacing: 0.005em;
  padding: 7px 14px;
  border-radius: 999px;
  border: 1px solid;
  white-space: nowrap;
  transition: transform .15s ease, box-shadow .15s ease;
}
.topic-pill:hover {
  transform: translateY(-1px);
  box-shadow: var(--shadow-sm);
}
.t-blue   { background: var(--blue-50);   color: var(--blue-900);   border-color: var(--blue-200); }
.t-amber  { background: var(--amber-50);  color: var(--amber-900);  border-color: var(--amber-200); }
.t-green  { background: var(--green-50);  color: var(--green-900);  border-color: var(--green-200); }
.t-purple { background: var(--purple-50); color: var(--purple-900); border-color: var(--purple-200); }

@media (max-width: 640px) {
  .topics-strip { padding: 28px 0 32px; }
  .topic-pill { font-size: 12.5px; padding: 6px 12px; }
}

/* ---------- Section title ---------- */

.section-title {
  font-family: var(--font-display);
  font-weight: 900;
  font-size: clamp(30px, 3.6vw, 46px);
  line-height: 1.08;
  letter-spacing: -0.02em;
  margin: 0 0 36px;
  color: var(--ink);
}
.section-title .muted {
  color: var(--muted);
  font-weight: 700;
}

/* ---------- Why grid (4 cards) ---------- */

.why { padding: 88px 0; background: var(--bg-soft); }

.why-grid {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 22px;
}
.card {
  position: relative;
  background: #fff;
  padding: 30px 28px 26px;
  border-radius: var(--radius-lg);
  box-shadow: var(--shadow-md);
  border-top: 6px solid;
  transition: transform .2s ease, box-shadow .2s ease;
}
.card:hover { transform: translateY(-4px); box-shadow: var(--shadow-lg); }

.card-tag {
  display: inline-block;
  font-size: 11px;
  font-weight: 800;
  letter-spacing: 0.16em;
  text-transform: uppercase;
  padding: 4px 10px;
  border-radius: 999px;
  margin-bottom: 12px;
}
.card-tag.mech { margin-top: 16px; background: rgba(12,18,38,0.06); color: var(--ink-soft); }

.card h3 {
  font-family: var(--font-body);
  font-weight: 800;
  font-size: 22px;
  line-height: 1.25;
  letter-spacing: -0.01em;
  margin: 0 0 6px;
  color: var(--ink);
}
.card p {
  margin: 0;
  color: var(--ink-soft);
  font-size: 16px;
}
.card-foot {
  margin-top: 20px;
  font-family: var(--font-mono);
  font-size: 12.5px;
  color: var(--muted);
  border-top: 1px dashed var(--hairline);
  padding-top: 12px;
}

.card-amber  { border-top-color: var(--amber-500);  }
.card-amber  .card-tag:not(.mech) { background: var(--amber-50);  color: var(--amber-900); }
.card-blue   { border-top-color: var(--blue-500);   }
.card-blue   .card-tag:not(.mech) { background: var(--blue-50);   color: var(--blue-900); }
.card-green  { border-top-color: var(--green-500);  }
.card-green  .card-tag:not(.mech) { background: var(--green-50);  color: var(--green-900); }
.card-purple { border-top-color: var(--purple-500); }
.card-purple .card-tag:not(.mech) { background: var(--purple-50); color: var(--purple-900); }

.why-coda {
  margin: 38px auto 0;
  max-width: 760px;
  text-align: center;
  font-family: var(--font-display);
  font-weight: 700;
  font-size: 22px;
  line-height: 1.45;
  color: var(--ink);
}

/* ---------- Volumes ---------- */

.volumes { padding: 100px 0 88px; background: var(--bg); }

.volume-grid {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 28px;
}
.volume {
  background: #fff;
  border-radius: var(--radius-lg);
  padding: 32px;
  box-shadow: var(--shadow-md);
  border: 1px solid var(--hairline);
  display: flex;
  flex-direction: column;
  position: relative;
  overflow: hidden;
}
.volume::after {
  content: "";
  position: absolute;
  top: 0; left: 0; right: 0;
  height: 6px;
}
.volume-vol1::after {
  background: linear-gradient(90deg, var(--blue-500), var(--purple-500));
}
.volume-vol2::after {
  background: linear-gradient(90deg, var(--green-500), var(--amber-500));
}
.volume-head {
  display: grid;
  grid-template-columns: auto 1fr;
  gap: 22px;
  align-items: center;
  margin-bottom: 18px;
}
.cover-mini {
  display: block;
  width: 130px;
  height: auto;
  border-radius: 8px;
  box-shadow:
    0 4px 10px rgba(12, 18, 38, 0.15),
    0 12px 28px rgba(12, 18, 38, 0.10);
  transition: transform .2s ease;
}
.cover-mini:hover { transform: translateY(-3px) rotate(-1deg); }
.volume-meta h3 {
  font-family: var(--font-display);
  font-weight: 900;
  font-size: 32px;
  margin: 0;
  letter-spacing: -0.02em;
  color: var(--ink);
}
.volume-sub {
  font-weight: 600;
  font-size: 16px;
  color: var(--ink-soft);
  margin: 4px 0 0;
}
.volume-chapters {
  font-family: var(--font-mono);
  font-size: 13px;
  color: var(--muted);
  margin: 8px 0 0;
}
.volume-pitch {
  font-size: 16.5px;
  color: var(--ink-soft);
  margin: 0 0 18px;
}
.volume-topics {
  list-style: none;
  padding: 0;
  margin: 0 0 24px;
  display: flex;
  flex-direction: column;
  gap: 10px;
}
.volume-topics li {
  position: relative;
  padding-left: 28px;
  font-size: 15.5px;
  color: var(--ink);
}
.volume-topics li::before {
  content: "";
  position: absolute;
  left: 0; top: 9px;
  width: 14px; height: 14px;
  border-radius: 50%;
  background: var(--green-500);
  box-shadow: 0 0 0 4px var(--green-50);
}
.volume-vol2 .volume-topics li::before {
  background: var(--purple-500);
  box-shadow: 0 0 0 4px var(--purple-50);
}
.volume-buy {
  display: flex;
  flex-wrap: wrap;
  gap: 12px;
  margin-top: auto;
}

/* ---------- Peek (SVG gallery) ---------- */

.peek {
  padding: 100px 0;
  background: linear-gradient(180deg, var(--bg) 0%, var(--bg-soft) 100%);
}
.peek-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(320px, 1fr));
  gap: 22px;
}
.peek-item {
  margin: 0;
  background: #fff;
  border-radius: var(--radius-lg);
  border: 1px solid var(--hairline);
  box-shadow: var(--shadow-sm);
  overflow: hidden;
  display: flex;
  flex-direction: column;
  transition: transform .2s ease, box-shadow .2s ease;
}
.peek-item:hover { transform: translateY(-4px); box-shadow: var(--shadow-md); }
.peek-svg {
  background: var(--bg-soft);
  padding: 18px;
  display: grid;
  place-items: center;
  border-bottom: 1px solid var(--hairline);
}
.peek-svg img {
  max-height: 240px;
  width: auto;
  max-width: 100%;
}
.peek-item figcaption {
  padding: 18px 22px 22px;
  font-size: 15px;
  color: var(--ink-soft);
}
.peek-chapter {
  display: inline-block;
  font-family: var(--font-mono);
  font-size: 11.5px;
  font-weight: 600;
  letter-spacing: 0.06em;
  text-transform: uppercase;
  color: var(--blue-900);
  background: var(--blue-50);
  padding: 3px 9px;
  border-radius: 999px;
  margin-bottom: 8px;
}
.peek-item figcaption strong {
  display: block;
  color: var(--ink);
  font-size: 16.5px;
  margin-bottom: 4px;
}
.peek-coda {
  margin-top: 48px;
  text-align: center;
  max-width: 720px;
  margin-left: auto;
  margin-right: auto;
}
.peek-coda p:first-child {
  font-family: var(--font-display);
  font-weight: 900;
  font-size: 28px;
  line-height: 1.2;
  color: var(--amber-900);
  margin: 0 0 14px;
}
.peek-coda p:last-child {
  font-size: 16.5px;
  color: var(--ink-soft);
  margin: 0;
}

/* ---------- Math section ---------- */

.math-section {
  padding: 100px 0;
  background:
    radial-gradient(800px 360px at 10% 0%, var(--c1-50), transparent 60%),
    radial-gradient(800px 360px at 90% 100%, var(--c3-50), transparent 60%),
    var(--bg);
}
.section-lede {
  max-width: 760px;
  margin: 0 auto 48px;
  text-align: center;
  font-size: 17.5px;
  line-height: 1.65;
  color: var(--ink-soft);
}
.math-grid {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 22px;
}
.math-card {
  display: grid;
  grid-template-columns: minmax(220px, 38%) 1fr;
  gap: 0;
  background: #fff;
  border: 1px solid var(--hairline);
  border-radius: var(--radius-lg);
  box-shadow: var(--shadow-sm);
  overflow: hidden;
  transition: transform .2s ease, box-shadow .2s ease;
}
.math-card:hover { transform: translateY(-4px); box-shadow: var(--shadow-md); }
.math-svg {
  background: var(--bg-soft);
  display: grid;
  place-items: center;
  padding: 22px;
  border-right: 1px solid var(--hairline);
}
.math-svg img { max-width: 100%; max-height: 200px; width: auto; height: auto; }
.math-body { padding: 24px 26px; display: flex; flex-direction: column; gap: 8px; }
.math-chapter {
  display: inline-block;
  font-family: var(--font-mono);
  font-size: 11px;
  font-weight: 700;
  letter-spacing: 0.16em;
  text-transform: uppercase;
  color: var(--c2-900);
  background: var(--c2-50);
  padding: 3px 9px;
  border-radius: 999px;
  align-self: flex-start;
}
.math-body h3 {
  font-family: var(--font-body);
  font-weight: 800;
  font-size: 19px;
  line-height: 1.25;
  letter-spacing: -0.01em;
  margin: 0;
  color: var(--ink);
}
.math-body p {
  margin: 0;
  font-size: 15px;
  line-height: 1.6;
  color: var(--ink-soft);
}
.math-coda {
  margin: 44px auto 0;
  max-width: 760px;
  text-align: center;
  padding: 22px 26px;
  background: var(--c3-50);
  border: 1px solid var(--c3-200);
  border-radius: var(--radius-md);
  font-size: 16px;
  color: var(--c3-900);
}
.math-coda strong { color: var(--c3-900); font-weight: 800; }

/* ---------- Q&A interview prep ---------- */

.qna-section {
  padding: 100px 0;
  background: var(--bg-soft);
  border-top: 1px solid var(--hairline);
  border-bottom: 1px solid var(--hairline);
}
.qna-grid {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 18px;
  max-width: 1100px;
  margin: 0 auto;
}
.qna-card {
  background: #fff;
  border: 1px solid var(--hairline);
  border-radius: var(--radius-md);
  box-shadow: var(--shadow-sm);
  overflow: hidden;
  transition: box-shadow .2s ease, border-color .2s ease;
}
.qna-card[open] {
  border-color: var(--c4-200);
  box-shadow: var(--shadow-md);
}
.qna-card summary {
  list-style: none;
  cursor: pointer;
  padding: 22px 24px;
  display: grid;
  grid-template-columns: 1fr auto;
  grid-template-areas:
    "tag toggle"
    "q   toggle";
  align-items: start;
  gap: 6px 14px;
}
.qna-card summary::-webkit-details-marker { display: none; }
.qna-tag {
  grid-area: tag;
  font-family: var(--font-mono);
  font-size: 10.5px;
  font-weight: 700;
  letter-spacing: 0.14em;
  text-transform: uppercase;
  color: var(--c4-900);
  background: var(--c4-50);
  padding: 3px 9px;
  border-radius: 999px;
  justify-self: start;
}
.qna-q {
  grid-area: q;
  font-weight: 700;
  font-size: 16px;
  line-height: 1.4;
  color: var(--ink);
  letter-spacing: -0.005em;
}
.qna-q code {
  font-family: var(--font-mono);
  font-size: 13.5px;
  background: var(--bg-soft);
  padding: 1px 6px;
  border-radius: 4px;
  color: var(--c2-900);
}
.qna-toggle {
  grid-area: toggle;
  align-self: center;
  flex-shrink: 0;
  width: 30px;
  height: 30px;
  border-radius: 50%;
  background: var(--c4-50);
  color: var(--c4-900);
  display: grid;
  place-items: center;
  font-family: var(--font-mono);
  font-weight: 700;
  font-size: 18px;
  transition: transform .2s ease, background .2s ease, color .2s ease;
}
.qna-card[open] .qna-toggle {
  background: var(--c4-700);
  color: #fff;
  transform: rotate(45deg);
}
.qna-a {
  padding: 0 24px 22px;
  border-top: 1px dashed var(--hairline);
  margin-top: 4px;
  padding-top: 16px;
}
.qna-a p {
  margin: 0;
  font-size: 15px;
  line-height: 1.65;
  color: var(--ink-soft);
}
.qna-a code {
  font-family: var(--font-mono);
  font-size: 13.5px;
  background: var(--bg);
  padding: 1px 6px;
  border-radius: 4px;
  color: var(--c2-900);
  border: 1px solid var(--hairline);
}
.qna-coda {
  margin: 44px auto 0;
  max-width: 760px;
  text-align: center;
  font-size: 16.5px;
  color: var(--ink-soft);
}

@media (max-width: 760px) {
  .math-grid { grid-template-columns: 1fr; }
  .math-card { grid-template-columns: 1fr; }
  .math-svg { border-right: 0; border-bottom: 1px solid var(--hairline); }
  .qna-grid { grid-template-columns: 1fr; }
}

/* ---------- Outcomes ---------- */

.outcomes {
  padding: 100px 0;
  background: var(--bg-deep);
  color: #fff;
  position: relative;
  overflow: hidden;
}
.outcomes::before {
  content: "";
  position: absolute;
  inset: 0;
  background:
    radial-gradient(700px 360px at 8% 12%, rgba(55,138,221,0.18), transparent 60%),
    radial-gradient(700px 360px at 92% 88%, rgba(127,119,221,0.18), transparent 60%);
  pointer-events: none;
}
.outcomes .container { position: relative; }
.outcomes .section-title { color: #fff; }

.outcome-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(260px, 1fr));
  gap: 16px;
}
.outcome {
  padding: 22px;
  border-radius: var(--radius-md);
  background: rgba(255,255,255,0.04);
  border: 1px solid rgba(255,255,255,0.08);
  transition: background .15s ease, transform .15s ease;
}
.outcome:hover { background: rgba(255,255,255,0.07); transform: translateY(-2px); }
.outcome-num {
  display: inline-block;
  font-family: var(--font-mono);
  font-size: 12px;
  font-weight: 700;
  letter-spacing: 0.1em;
  color: var(--amber-200);
  margin-bottom: 8px;
}
.outcome p {
  margin: 0;
  font-size: 16px;
  color: rgba(255,255,255,0.92);
}

/* ---------- Author ---------- */

.author {
  padding: 100px 0;
  background: var(--bg-soft);
  border-top: 1px solid var(--hairline);
  border-bottom: 1px solid var(--hairline);
}
.author-grid {
  display: grid;
  grid-template-columns: 280px 1fr;
  gap: 60px;
  align-items: center;
}
.author-photo {
  position: relative;
}
.author-photo img {
  width: 100%;
  height: auto;
  border-radius: 50%;
  background: #fff;
  box-shadow:
    0 0 0 8px #fff,
    0 0 0 9px var(--hairline),
    0 18px 40px rgba(12, 18, 38, 0.14);
  transition: transform .3s ease;
}
.author-photo::before {
  content: "";
  position: absolute;
  inset: -22px;
  border-radius: 50%;
  background:
    radial-gradient(60% 60% at 30% 30%, rgba(55, 138, 221, 0.18), transparent 70%),
    radial-gradient(60% 60% at 70% 70%, rgba(127, 119, 221, 0.14), transparent 70%);
  z-index: -1;
}
.author-photo:hover img { transform: rotate(-1deg); }

.author-bio .kicker { margin-bottom: 16px; }
.author-name {
  font-family: var(--font-display);
  font-weight: 900;
  font-size: clamp(34px, 4vw, 48px);
  line-height: 1.05;
  letter-spacing: -0.02em;
  margin: 0 0 18px;
  color: var(--ink);
}
.author-roles {
  display: flex;
  flex-wrap: wrap;
  gap: 10px;
  margin-bottom: 22px;
}
.role-pill {
  display: inline-flex;
  align-items: center;
  padding: 7px 14px;
  border-radius: 999px;
  font-size: 13px;
  font-weight: 700;
  letter-spacing: 0.01em;
  border: 1px solid transparent;
}
.role-current {
  background: var(--green-50);
  color: var(--green-900);
  border-color: var(--green-200);
}
.role-past {
  background: var(--blue-50);
  color: var(--blue-900);
  border-color: var(--blue-200);
}
.author-text {
  font-size: 16.5px;
  line-height: 1.65;
  color: var(--ink-soft);
  max-width: 64ch;
  margin: 0 0 14px;
}
.author-text:last-of-type { margin-bottom: 22px; }
.author-cta { margin: 0; }

/* ---------- FAQ ---------- */

.faq {
  padding: 100px 0;
  background: var(--bg);
  border-top: 1px solid var(--hairline);
}
.faq-list {
  max-width: 880px;
  margin: 0 auto;
  display: flex;
  flex-direction: column;
  gap: 12px;
}
.faq-item {
  background: #fff;
  border: 1px solid var(--hairline);
  border-radius: var(--radius-md);
  padding: 0;
  box-shadow: var(--shadow-sm);
  transition: box-shadow .2s ease, border-color .2s ease;
}
.faq-item[open] {
  border-color: var(--blue-200);
  box-shadow: var(--shadow-md);
}
.faq-item summary {
  list-style: none;
  cursor: pointer;
  padding: 22px 26px;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 16px;
  font-weight: 700;
  font-size: 17px;
  color: var(--ink);
}
.faq-item summary::-webkit-details-marker { display: none; }
.faq-item summary::after {
  content: "+";
  flex-shrink: 0;
  width: 28px;
  height: 28px;
  border-radius: 50%;
  background: var(--blue-50);
  color: var(--blue-900);
  display: grid;
  place-items: center;
  font-family: var(--font-mono);
  font-weight: 700;
  font-size: 18px;
  transition: transform .2s ease, background .2s ease;
}
.faq-item[open] summary::after {
  content: "−";
  background: var(--blue-900);
  color: #fff;
  transform: rotate(180deg);
}
.faq-item summary:hover { color: var(--blue-900); }
.faq-q {
  flex: 1;
  letter-spacing: -0.005em;
}
.faq-item p {
  margin: 0;
  padding: 0 26px 24px;
  color: var(--ink-soft);
  font-size: 16px;
  line-height: 1.65;
  max-width: 70ch;
}
.faq-item p a { font-weight: 600; color: var(--blue-700); }
.faq-item p a:hover { color: var(--blue-900); text-decoration: underline; }

/* ---------- Buy ---------- */

.buy {
  padding: 100px 0 120px;
  background:
    radial-gradient(900px 420px at 10% 0%, var(--amber-50), transparent 60%),
    radial-gradient(900px 420px at 90% 100%, var(--purple-50), transparent 60%),
    var(--bg);
}
.buy-card {
  background: #fff;
  border-radius: 28px;
  padding: 56px 48px;
  box-shadow: var(--shadow-lg);
  border: 1px solid var(--hairline);
  text-align: center;
  position: relative;
  overflow: hidden;
}
.buy-card::before {
  content: "";
  position: absolute;
  inset: -2px;
  border-radius: 28px;
  padding: 2px;
  background: linear-gradient(135deg, var(--amber-500), var(--blue-500), var(--green-500), var(--purple-500));
  -webkit-mask:
    linear-gradient(#000 0 0) content-box,
    linear-gradient(#000 0 0);
          mask:
    linear-gradient(#000 0 0) content-box,
    linear-gradient(#000 0 0);
  -webkit-mask-composite: xor;
          mask-composite: exclude;
  pointer-events: none;
}
.buy-kicker {
  font-family: var(--font-mono);
  font-size: 13px;
  font-weight: 700;
  letter-spacing: 0.16em;
  text-transform: uppercase;
  color: var(--blue-900);
  margin: 0 0 16px;
}
.buy-card h2 {
  font-family: var(--font-display);
  font-weight: 900;
  font-size: clamp(28px, 3.4vw, 42px);
  line-height: 1.1;
  letter-spacing: -0.02em;
  margin: 0 0 16px;
  color: var(--ink);
}
.buy-pitch {
  max-width: 60ch;
  margin: 0 auto 36px;
  font-size: 17px;
  color: var(--ink-soft);
}
.buy-grid {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 22px;
  margin-bottom: 36px;
  text-align: left;
}
.buy-vol {
  padding: 24px;
  border-radius: var(--radius-md);
  background: var(--bg-soft);
  border: 1px solid var(--hairline);
  display: flex;
  flex-direction: column;
  gap: 10px;
}
.buy-vol h3 {
  font-family: var(--font-display);
  font-weight: 900;
  font-size: 24px;
  margin: 0;
  color: var(--ink);
}
.buy-vol p {
  margin: 0 0 8px;
  color: var(--ink-soft);
  font-size: 15px;
}
.buy-vol .btn { align-self: flex-start; }
.buy-foot {
  font-family: var(--font-display);
  font-style: italic;
  font-size: 19px;
  color: var(--ink);
  max-width: 60ch;
  margin: 0 auto;
  line-height: 1.4;
}

/* ---------- Footer ---------- */

.site-foot {
  padding: 28px 0 36px;
  background: var(--bg-deep);
  color: rgba(255,255,255,0.72);
  font-size: 14px;
}
.foot-row {
  display: flex;
  justify-content: space-between;
  flex-wrap: wrap;
  gap: 12px;
}
.foot-sep { margin: 0 8px; color: rgba(255,255,255,0.35); }
.site-foot a { color: var(--amber-200); }
.site-foot a:hover { color: var(--amber-500); }

/* ---------- Responsive ---------- */

@media (max-width: 960px) {
  .hero { padding: 60px 0 70px; }
  .hero-grid { grid-template-columns: 1fr; gap: 40px; }
  .hero-art { max-width: 480px; margin: 0 auto; }
  .why-grid { grid-template-columns: 1fr; }
  .volume-grid { grid-template-columns: 1fr; }
  .buy-grid { grid-template-columns: 1fr; }
  .buy-card { padding: 40px 28px; }
  .author-grid { grid-template-columns: 1fr; gap: 32px; text-align: center; }
  .author-photo { width: 220px; margin: 0 auto; }
  .author-roles { justify-content: center; }
  .author-text { margin-left: auto; margin-right: auto; }
}

@media (max-width: 640px) {
  .topnav { gap: 16px; }
  .topnav a:not(.nav-cta) { display: none; }
  .brand-text { display: none; }
  .container { padding: 0 18px; }
  .volume { padding: 24px; }
  .volume-head { grid-template-columns: 1fr; gap: 18px; }
  .cover-mini { width: 110px; }
  .display { font-size: 38px; }
  .author { padding: 70px 0; }
  .author-photo { width: 180px; }
}