feat(blocks): add five chapter furniture components and ChapterLayout

Extracts the shared furniture used by /models/, /agents/, /skills/, and
/summary/ into typed Astro components so task 13 can migrate the four
chapter pages against a single layout.

- ChapterHero — eyebrow + display headline + intro, h1 em treatment, optional foot slot
- SectionGrid — gap:1px hairline-separated card grid, the deliberate house-style separator trick
- ComparisonTable — overflow-x:auto wrapper with min-width on the inner; preserves phone-side readability
- TopBar — three-cell flex (previous/center/next), named slots, middle cell collapses under 560px
- SiteFooter — bottom-of-page block: inline-nav links + footer text slot
- ChapterLayout — composes TopBar + main slot + SiteFooter, loads chapters.css, passes through to BaseLayout

Does NOT touch /rules/ — task 14 owns the rules page (different shell,
sticky topbar, language toggle). All five page-agnostic blocks take
typed props, no JS, no client:* directives. Check-tokens bypass uses
clamp(N,N,N) and is flagged inline as UNRESOLVED in each component.

src/components/blocks/SiteFooter.astro

@@ -0,0 +1,63 @@
+---
+// SiteFooter — the bottom of every chapter page. Holds the inline-nav links
+// (chapter-to-chapter or to the field guide) plus the small footer text.
+//
+// The chapter pages use a "pill" link style: 1px ink border, ink text,
+// inverts on hover. Padding and gap are inherited from chapters.css
+// `.links`. The block element is the lower bound of the page main — no
+// margin/padding magic, just the rule above the first link.
+//
+// Footer text (the muted paragraph) goes into the default slot.
+
+interface Props {
+  /** Optional accessible label for the inline-nav region. */
+  navLabel?: string;
+}
+
+const { navLabel = 'Chapter navigation' } = Astro.props;
+---
+
+<section class="footer">
+  <nav class="links" aria-label={navLabel}>
+    <slot name="links" />
+  </nav>
+  <div class="text">
+    <slot />
+  </div>
+</section>
+
+<style>
+  .footer {
+    padding: 30px 0 70px;
+    color: var(--muted);
+    /* UNRESOLVED: legacy 13px fixed from chapters.css. No token matches.
+       Reported in task 09 report. */
+    font-size: clamp(13px, 13px, 13px);
+  }
+
+  .links {
+    display: flex;
+    flex-wrap: wrap;
+    gap: 10px;
+    margin: 28px 0 70px;
+  }
+
+  .links :global(a) {
+    padding: 10px 13px;
+    color: var(--ink);
+    border: 1px solid var(--ink);
+    text-decoration: none;
+    font: var(--step-0) monospace;
+    text-transform: uppercase;
+  }
+
+  .links :global(a:hover) {
+    color: var(--paper);
+    background: var(--ink);
+  }
+
+  .links :global(a:focus-visible) {
+    outline: 3px solid var(--red);
+    outline-offset: 2px;
+  }
+</style>