:root {
  --ink:        #1a1a1a;
  --ink-soft:   #3d3d3d;
  --muted:      #6b6b6b;
  --faint:      #9a9a9a;
  --line:       #e4e2dd;
  --line-soft:  #efede8;
  --paper:      #faf9f6;
  --paper-2:    #f4f2ec;
  --card:       #ffffff;
  --accent:     #174ea6;   /* deep scholarly blue (from the paper) */
  --slate:      #5c7a99;   /* muted slate accent */
  --serif: "Newsreader", Georgia, "Times New Roman", serif;
  --sans:  "Inter", -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
  --wrap: 1080px;
}

* { box-sizing: border-box; }

html { scroll-behavior: smooth; }

body {
  margin: 0;
  font-family: var(--sans);
  color: var(--ink);
  background: var(--paper);
  line-height: 1.65;
  -webkit-font-smoothing: antialiased;
  text-rendering: optimizeLegibility;
}

.wrap {
  max-width: var(--wrap);
  margin: 0 auto;
  padding: 0 32px;
}

a { color: inherit; text-decoration: none; }

/* ---------------- NAV ---------------- */
.nav {
  position: sticky;
  top: 0;
  z-index: 50;
  background: rgba(250, 249, 246, 0.82);
  backdrop-filter: saturate(160%) blur(12px);
  border-bottom: 1px solid transparent;
  transition: border-color .3s ease, background .3s ease;
}
.nav.scrolled { border-color: var(--line); }
.nav-inner {
  display: flex;
  align-items: center;
  justify-content: space-between;
  height: 66px;
}
.brand { display: flex; align-items: center; gap: 10px; }
.brand-mark { width: 30px; height: 30px; object-fit: contain; mix-blend-mode: multiply; }
.brand-name {
  font-family: var(--serif);
  font-weight: 600;
  font-size: 1.2rem;
  letter-spacing: .02em;
}
.nav-links { display: flex; align-items: center; gap: 28px; }
.nav-links a {
  font-size: .86rem;
  color: var(--muted);
  letter-spacing: .01em;
  transition: color .2s ease;
}
.nav-links a:hover { color: var(--ink); }
.nav-cta {
  border: 1px solid var(--line);
  padding: 7px 16px;
  border-radius: 999px;
  color: var(--ink) !important;
}
.nav-cta:hover { border-color: var(--ink); }

@media (max-width: 720px) {
  .nav-links a:not(.nav-cta) { display: none; }
}

/* ---------------- HERO ---------------- */
.hero {
  position: relative;
  padding: 96px 0 88px;
  border-bottom: 1px solid var(--line);
  background:
    radial-gradient(ellipse 60% 80% at 78% 20%, rgba(23,78,166,.05), transparent 60%),
    var(--paper);
}
.hero-inner {
  display: grid;
  grid-template-columns: 1.15fr 0.85fr;
  gap: 56px;
  align-items: center;
}
.eyebrow {
  font-size: .78rem;
  letter-spacing: .22em;
  text-transform: uppercase;
  color: var(--slate);
  margin: 0 0 18px;
  font-weight: 500;
}
.hero-title {
  font-family: var(--serif);
  font-weight: 500;
  font-size: clamp(2.3rem, 6.2vw, 4.1rem);
  line-height: 1.0;
  letter-spacing: -0.015em;
  margin: 0 0 12px;
  white-space: nowrap;
}
.morph { white-space: nowrap; }
/* Morphing title: AI4[X → Productivity → Science → AI] */
.hero-title .ai4 { color: var(--ink); }
.morph {
  display: inline-block;
  position: relative;
  color: var(--accent);
  vertical-align: baseline;
}
.morph-word {
  display: inline-block;
  transition: opacity .45s ease, transform .45s cubic-bezier(.22,.61,.36,1), filter .45s ease;
  will-change: opacity, transform, filter;
}
.morph-word.out {
  opacity: 0;
  transform: translateY(-0.14em);
  filter: blur(4px);
}
.morph-word.in {
  opacity: 0;
  transform: translateY(0.14em);
  filter: blur(4px);
}
.hero-formula {
  font-family: var(--serif);
  font-style: italic;
  font-size: 1.02rem;
  color: var(--muted);
  margin: 6px 0 26px;
  letter-spacing: .01em;
}
.hero-formula .var { color: var(--accent); font-weight: 500; }
@media (prefers-reduced-motion: reduce) {
  .morph-word { transition: none; }
}

.hero-sub {
  font-size: 1.14rem;
  color: var(--ink-soft);
  max-width: 34em;
  margin: 0 0 34px;
}
.hero-actions { display: flex; gap: 14px; flex-wrap: wrap; }
.btn {
  display: inline-flex;
  align-items: center;
  font-size: .92rem;
  font-weight: 500;
  padding: 12px 24px;
  border-radius: 999px;
  border: 1px solid transparent;
  transition: all .2s ease;
}
.btn-primary { background: var(--ink); color: var(--paper); }
.btn-primary:hover { background: var(--accent); transform: translateY(-1px); }
.btn-ghost { border-color: var(--line); color: var(--ink); }
.btn-ghost:hover { border-color: var(--ink); }
.hero-meta {
  margin-top: 30px;
  font-size: .84rem;
  color: var(--faint);
  display: flex;
  gap: 10px;
  align-items: center;
  letter-spacing: .02em;
}
.hero-meta .dot { color: var(--line); }

.hero-figure { display: flex; justify-content: center; }
.hero-figure img {
  width: 100%;
  max-width: 380px;
  object-fit: contain;
  mix-blend-mode: multiply;
  filter: grayscale(100%) contrast(1.02);
}

@media (max-width: 860px) {
  .hero-inner { grid-template-columns: 1fr; gap: 40px; }
  .hero-figure { order: -1; }
  .hero-figure img { max-width: 260px; }
}

/* ---------------- SECTIONS ---------------- */
.section { padding: 92px 0; border-bottom: 1px solid var(--line); }
.section-head { margin-bottom: 48px; max-width: 44em; }
.section-index {
  display: inline-block;
  font-family: var(--serif);
  font-size: .9rem;
  color: var(--slate);
  letter-spacing: .1em;
  margin-bottom: 14px;
}
.section-head h2 {
  font-family: var(--serif);
  font-weight: 500;
  font-size: clamp(1.9rem, 4vw, 2.7rem);
  line-height: 1.12;
  letter-spacing: -0.01em;
  margin: 0;
}
.section-desc {
  margin: 18px 0 0;
  font-size: 1.05rem;
  color: var(--muted);
  max-width: 40em;
}

/* ---------------- VISION ---------------- */
.vision-grid {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 44px;
}
.lede {
  font-family: var(--serif);
  font-size: 1.4rem;
  line-height: 1.45;
  color: var(--ink);
  margin: 0;
}
.vision-grid p:not(.lede) { color: var(--ink-soft); font-size: 1.02rem; margin: 0; }
.vision em { font-style: italic; color: var(--accent); }
@media (max-width: 760px) {
  .vision-grid { grid-template-columns: 1fr; gap: 26px; }
  .lede { font-size: 1.22rem; }
}

/* ---------------- SERIES ---------------- */
.part-list { list-style: none; margin: 0; padding: 0; }
.part {
  display: grid;
  grid-template-columns: 220px 1fr;
  gap: 44px;
  padding: 40px 0;
  border-top: 1px solid var(--line);
  transition: background .3s ease;
}
.part:last-child { border-bottom: 1px solid var(--line); }
.part:hover { background: linear-gradient(90deg, rgba(23,78,166,.025), transparent 40%); }
.part-order { display: flex; flex-direction: column; gap: 12px; }
.numeral {
  font-family: var(--serif);
  font-size: 3.4rem;
  font-weight: 400;
  line-height: 1;
  color: var(--ink);
}
.order-label {
  font-size: .74rem;
  text-transform: uppercase;
  letter-spacing: .16em;
  color: var(--slate);
  font-weight: 500;
}
.part-body h3 {
  font-family: var(--serif);
  font-weight: 600;
  font-size: 1.55rem;
  margin: 0 0 12px;
  letter-spacing: -.005em;
}
.part-body p { margin: 0 0 18px; color: var(--ink-soft); max-width: 46em; }
.part-body strong { color: var(--ink); font-weight: 600; }
.part-links {
  display: flex;
  flex-wrap: wrap;
  gap: 10px 22px;
  align-items: center;
}
.part-link {
  font-size: .9rem;
  font-weight: 500;
  color: var(--accent);
  transition: gap .2s ease;
}
.part-link:hover span { padding-left: 5px; }
.part-link span { transition: padding .2s ease; }
.part-link.muted { color: var(--faint); cursor: default; }
@media (max-width: 700px) {
  .part { grid-template-columns: 1fr; gap: 16px; }
  .numeral { font-size: 2.6rem; }
}

/* ---------------- PART I INSIGHTS ---------------- */

.insight-sub {
  font-size: .76rem;
  text-transform: uppercase;
  letter-spacing: .18em;
  color: var(--slate);
  font-weight: 600;
  margin: 46px 0 22px;
}
.insight-sub:first-of-type { margin-top: 0; }

.definition {
  margin: 0 0 8px;
  padding: 0;
  border-left: 2px solid var(--accent);
}
.definition blockquote {
  margin: 0;
  padding: 6px 0 6px 26px;
  font-family: var(--serif);
  font-size: clamp(1.3rem, 2.6vw, 1.75rem);
  line-height: 1.4;
  color: var(--ink);
}
.definition em { font-style: italic; color: var(--accent); }
.def-term { font-weight: 600; }

/* Two mechanisms */
.mech-grid { display: grid; grid-template-columns: 1fr 1fr; gap: 20px; }
.mech {
  padding: 26px 26px 24px;
  border: 1px solid var(--line);
  border-radius: 12px;
  background: var(--card);
}
.mech-mark {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 34px; height: 34px;
  border-radius: 50%;
  background: var(--paper-2);
  color: var(--accent);
  font-size: 1.1rem;
  margin-bottom: 14px;
}
.mech h4 { margin: 0 0 8px; font-family: var(--serif); font-size: 1.25rem; font-weight: 600; }
.mech p { margin: 0; color: var(--ink-soft); font-size: .98rem; }
@media (max-width: 640px) { .mech-grid { grid-template-columns: 1fr; } }

/* Evolution ladder */
.ladder {
  list-style: none;
  margin: 0;
  padding: 0;
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 0;
  counter-reset: rung;
}
.ladder li {
  position: relative;
  padding: 22px 20px 22px 0;
  border-top: 2px solid var(--line);
}
.ladder li::before {
  content: "";
  position: absolute;
  top: -5px; left: 0;
  width: 8px; height: 8px;
  border-radius: 50%;
  background: var(--slate);
}
.ladder li:first-child::before { background: var(--accent); }
.rung {
  display: block;
  font-family: var(--serif);
  font-size: .82rem;
  color: var(--slate);
  letter-spacing: .1em;
  margin-bottom: 6px;
}
.rung-name { font-size: 1.02rem; color: var(--ink); font-weight: 500; line-height: 1.3; display: block; }
@media (max-width: 760px) {
  .ladder { grid-template-columns: 1fr 1fr; }
}
@media (max-width: 440px) {
  .ladder { grid-template-columns: 1fr; }
}

/* Diffusion tiers */
.diffusion { display: grid; grid-template-columns: repeat(3, 1fr); gap: 16px; }
.tier {
  padding: 22px 22px 20px;
  border-radius: 12px;
  border: 1px solid var(--line);
}
.tier-label {
  display: inline-block;
  font-size: .72rem;
  text-transform: uppercase;
  letter-spacing: .14em;
  font-weight: 700;
  padding: 4px 12px;
  border-radius: 999px;
  margin-bottom: 14px;
}
.tier p { margin: 0; color: var(--ink-soft); font-size: .96rem; line-height: 1.55; }
.tier-1 { background: linear-gradient(180deg, rgba(92,122,153,.05), transparent); }
.tier-1 .tier-label { background: rgba(92,122,153,.14); color: #45607a; }
.tier-2 { background: linear-gradient(180deg, rgba(176,96,0,.05), transparent); }
.tier-2 .tier-label { background: rgba(176,96,0,.12); color: #9a5400; }
.tier-3 { background: linear-gradient(180deg, rgba(23,78,166,.06), transparent); }
.tier-3 .tier-label { background: rgba(23,78,166,.12); color: var(--accent); }
@media (max-width: 760px) { .diffusion { grid-template-columns: 1fr; } }

/* Challenge chips */
.chips { list-style: none; margin: 0; padding: 0; display: flex; flex-wrap: wrap; gap: 10px; }
.chips li {
  font-size: .9rem;
  color: var(--ink-soft);
  padding: 8px 16px;
  border: 1px solid var(--line);
  border-radius: 999px;
  background: var(--card);
  transition: border-color .2s ease, color .2s ease;
}
.chips li:hover { border-color: var(--accent); color: var(--accent); }

/* ---------------- CITE ---------------- */
.cite-card {
  position: relative;
  background: var(--paper-2);
  border: 1px solid var(--line);
  border-radius: 12px;
  overflow: hidden;
}
.cite-card pre {
  margin: 0;
  padding: 28px 30px;
  font-family: "SF Mono", ui-monospace, "JetBrains Mono", Menlo, monospace;
  font-size: .86rem;
  line-height: 1.7;
  color: var(--ink-soft);
  overflow-x: auto;
}
.copy-btn {
  position: absolute;
  top: 16px;
  right: 16px;
  background: var(--card);
  border: 1px solid var(--line);
  color: var(--muted);
  font-family: var(--sans);
  font-size: .78rem;
  padding: 6px 14px;
  border-radius: 999px;
  cursor: pointer;
  transition: all .2s ease;
}
.copy-btn:hover { border-color: var(--ink); color: var(--ink); }
.copy-btn.copied { color: var(--accent); border-color: var(--accent); }

/* ---------------- FOOTER ---------------- */
.footer { padding: 46px 0; background: var(--paper); }
.footer-inner {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 24px;
  flex-wrap: wrap;
}
.footer-brand { display: flex; align-items: center; gap: 12px; }
.footer-mark { width: 34px; height: 34px; object-fit: contain; mix-blend-mode: multiply; }
.footer-name { margin: 0; font-family: var(--serif); font-weight: 600; font-size: 1.05rem; }
.footer-tag { margin: 0; font-size: .8rem; color: var(--faint); letter-spacing: .04em; }
.footer-links { display: flex; gap: 22px; }
.footer-links a { font-size: .88rem; color: var(--muted); transition: color .2s ease; }
.footer-links a:hover { color: var(--ink); }
.footer-copy { margin: 0; font-size: .82rem; color: var(--faint); }

/* ---------------- REVEAL ANIMATION ---------------- */
.reveal {
  opacity: 0;
  transform: translateY(16px);
  transition: opacity .7s ease, transform .7s cubic-bezier(.22,.61,.36,1);
}
.reveal.in { opacity: 1; transform: none; }
@media (prefers-reduced-motion: reduce) {
  .reveal { opacity: 1; transform: none; transition: none; }
  html { scroll-behavior: auto; }
}