/* ===========================================================
   Backend Principle, page shell
   -----------------------------------------------------------
   One layout for all 24 chapters: sticky table of contents on
   the left, article on the right, drawer on small screens.
   Chapters supply content styling only; everything structural
   lives here.
   =========================================================== */

/* ---------- Shell ---------- */
.bfp-shell {
  display: grid;
  grid-template-columns: 280px minmax(0, 1fr);
  max-width: 1440px;
  margin: 0 auto;
  align-items: start;
}

/* ---------- Table of contents ---------- */
.bfp-toc {
  position: sticky;
  top: 0;
  align-self: start;
  height: 100vh;
  overflow-y: auto;
  overscroll-behavior: contain;
  padding: 38px 20px 96px 26px;
  border-right: 1px solid var(--bfp-line);
  background: linear-gradient(180deg, var(--bfp-paper-2), var(--bfp-paper));
}

.bfp-toc::-webkit-scrollbar { width: 8px; }
.bfp-toc::-webkit-scrollbar-track { background: transparent; }
.bfp-toc::-webkit-scrollbar-thumb {
  background: var(--bfp-line-strong);
  border-radius: 4px;
  border: 2px solid transparent;
  background-clip: content-box;
}

.bfp-brand {
  font-family: var(--bfp-font-display);
  font-weight: 900;
  font-size: 23px;
  line-height: 1.1;
  letter-spacing: -.015em;
  color: var(--bfp-ink);
}
.bfp-brand-slash { color: var(--bfp-accent); font-weight: 700; margin: 0 3px 0 6px; }
.bfp-brand-lens {
  display: block;
  font-style: italic;
  font-weight: 600;
  font-size: 19px;
  color: var(--bfp-ink);
}

.bfp-brand-sub {
  font-family: var(--bfp-font-mono);
  font-size: 10px;
  letter-spacing: .22em;
  text-transform: uppercase;
  color: var(--bfp-ink-faint);
  margin-top: 11px;
}

.bfp-toc-rule {
  height: 1px;
  background: var(--bfp-line);
  margin: 22px 0 4px;
}

.bfp-toc-nav { display: flex; flex-direction: column; }

/* chapters style bare `a` for prose links; the contents list opts out */
.bfp-toc a,
.bfp-toc a:hover,
.bfp-toc a:visited {
  border: 0;
  border-left-style: solid;
  box-shadow: none;
  background-image: none;
  text-decoration: none;
}

.bfp-toc-part {
  font-family: var(--bfp-font-mono);
  font-size: 9.5px;
  letter-spacing: .2em;
  text-transform: uppercase;
  color: var(--bfp-ink-faint);
  margin: 22px 0 7px;
  padding-left: 10px;
}
.bfp-toc-part:first-child { margin-top: 14px; }

.bfp-toc-link {
  display: flex;
  align-items: baseline;
  gap: 10px;
  padding: 6px 8px 6px 8px;
  margin-left: 2px;
  border-left: 2px solid transparent;
  border-radius: 0 7px 7px 0;
  text-decoration: none;
  color: var(--bfp-ink-soft);
  font-size: 13.5px;
  line-height: 1.35;
  transition: color .15s ease, background .15s ease, border-color .15s ease;
}
.bfp-toc-link:hover {
  color: var(--bfp-ink);
  background: rgba(var(--accent-rgb), .08);
  border-left-color: var(--bfp-accent);
}
.bfp-toc-link.is-active {
  color: var(--bfp-accent);
  border-left-color: var(--bfp-accent);
  background: rgba(var(--accent-rgb), .12);
  font-weight: 700;
}
.bfp-toc-num {
  font-family: var(--bfp-font-mono);
  font-size: 10.5px;
  font-weight: 600;
  color: var(--bfp-accent);
  flex: none;
  min-width: 17px;
}
.bfp-toc-text { min-width: 0; }
.bfp-toc-link.is-sub {
  padding-left: 30px;
  font-size: 12.8px;
}
.bfp-toc-link.is-sub .bfp-toc-num { display: none; }

/* ---------- Article ---------- */
.bfp-content {
  padding: 64px clamp(22px, 4.5vw, 80px) 150px;
  max-width: 1040px;
  min-width: 0;
}

/* ---------- Chapter sign-off ---------- */
.bfp-footer {
  margin-top: 96px;
  padding-top: 22px;
  border-top: 2px solid var(--bfp-accent);
  font-family: var(--bfp-font-mono);
  font-size: 11.5px;
  letter-spacing: .06em;
  line-height: 1.8;
  color: var(--bfp-ink-faint);
}

/* ---------- Drawer toggle (small screens only) ---------- */
.bfp-toc-toggle {
  display: none;
  position: fixed;
  top: 14px;
  left: 14px;
  z-index: 9996;
  align-items: center;
  gap: 8px;
  padding: 9px 15px;
  border: 1px solid var(--bfp-line-strong);
  border-radius: 9px;
  background: rgba(243, 237, 226, .92);
  backdrop-filter: blur(10px);
  -webkit-backdrop-filter: blur(10px);
  color: var(--bfp-ink-soft);
  font-family: var(--bfp-font-mono);
  font-size: 12px;
  font-weight: 600;
  cursor: pointer;
  box-shadow: 0 6px 18px -12px rgba(40, 30, 20, .5);
}
.bfp-toc-toggle:hover { color: var(--bfp-accent); border-color: var(--bfp-accent); }

.bfp-toc-scrim {
  display: none;
  position: fixed;
  inset: 0;
  z-index: 9996;
  background: rgba(33, 30, 26, .38);
  backdrop-filter: blur(2px);
  opacity: 0;
  transition: opacity .2s ease;
}
.bfp-toc-scrim.is-open { opacity: 1; }

/* ---------- Responsive ---------- */
@media (max-width: 1000px) {
  .bfp-shell { display: block; }
  .bfp-toc-toggle { display: inline-flex; }
  .bfp-toc-scrim.is-open { display: block; }

  .bfp-toc {
    position: fixed;
    top: 0;
    left: 0;
    bottom: 0;
    height: 100dvh;
    width: min(330px, 86vw);
    z-index: 9997;
    padding: 62px 18px 40px 22px;
    border-right: 1px solid var(--bfp-line-strong);
    box-shadow: 18px 0 46px -30px rgba(40, 30, 20, .7);
    transform: translateX(-102%);
    transition: transform .24s cubic-bezier(.4, 0, .2, 1);
  }
  .bfp-toc.is-open { transform: none; }

  .bfp-content { padding: 74px 22px 130px; }
}

@media (max-width: 560px) {
  .bfp-content { padding: 70px 18px 120px; }
  .bfp-brand { font-size: 20px; }
  .bfp-brand-lens { font-size: 17px; }
}

@media print {
  .bfp-shell { display: block; }
  .bfp-toc, .bfp-toc-toggle, .bfp-toc-scrim { display: none !important; }
  .bfp-content { padding: 0; }
}