feat(gallery): detail-page navigation, code ergonomics, image polish

[TMHSDigital]

Summary

PR 1 of 3 in the website QoL pass (gallery first, then landing).

• Navigation: prev/next within kind (examples ↔ examples, showcase ↔ showcase, no wrap) at the top and foot of every detail page; ←/→ keys page (disabled while typing or while the lightbox is open); tag list linking to ../#tag=<t>; a Related strip of 3 entries ranked by shared topic tags (showcase excluded), ties broken by gallery order.
• Code: line-number gutter (separate aria-hidden pre; highlight_python untouched); listings > 120 lines collapse with "Show all N lines" (JS-only; no-JS renders in full); copy button on every README and source block via one copyText() with an execCommand fallback and a visible "Copy failed" state; README h1–h3 get deduped slug ids + hover # anchors, with ids reserved against page ids.
• Images: cards carry width/height (+ height:auto) so the grid reserves space before load; first 4 cards eager; detail hero fetchpriority="high"; lightbox closes on backdrop / Close / Esc only, image click toggles native size.

Evidence

Proven by live run (local python -m http.server -d docs, Playwright/Chromium):

• build_gallery.py run twice → diff -rq identical (deterministic; the validate.yml drift gate will hold).
• bmesh-gear: 372-line source collapsed, 2 copy buttons injected, anchors run, exit-codes, pager driver-wave ← → shader-node-group, related = uv-layer-grid, collision-hull-proxy, mesh-hygiene-audit.
• Lightbox: opens; image click toggles .native without closing; backdrop click closes; Close button closes. Copy → "Copied".
• ArrowRight on bmesh-gear navigated to shader-node-group.
• 375px: scrollWidth 360 ≤ 375 (no horizontal overflow); key hint hidden.
• Index: all 88 card images measure 188px tall at 1440px before and after load; exactly 4 eager.
• Caught and fixed during the run: the height attribute overrode aspect-ratio (cards rendered 720px tall) until height:auto was added.

By inspection only: the execCommand fallback path (requires a non-secure origin); showcase edge pagers (shipping-crate has no prev, book-trolley no next) read from generated HTML.

🤖 Generated with Claude Code