Simplify gates; fix focus, dark figures, footer, print; replace ar5iv figures

[bits-bytes-nn]

Adversarial review loop (code/architecture, tests/CI/docs, UI/UX/visuals), 3 rounds; round 3: 0 MAJOR in both reviews.

Fixed on the live site

• ar5iv figures: ar5iv now returns a "NO IMAGE AVAILABLE" PNG with HTTP 200 for 59 of 89 figures, so no link check saw it. 46 figures now point at arxiv.org/html (same bytes); 42 from papers without an HTML version (Llama 2, Constitutional AI, QLoRA, Mistral 7B, LLaMA, Mixtral, CoT) are rendered from the arXiv sources into assets/images/<slug>-arxiv-xN.png and checked against their captions. Two ar5iv citation links in QLoRA point at the Elo rating article instead.
• Llama 2 post: three images stood in for tables (Tables 3, 6, 7) and are removed; the reward-model scaling figure moves to the paragraph that describes it; the false-refusal paragraph gets Figure 33; two captions match their figures.
• Focus ring visible on zoomable figures; dark mode gives transparent figures a white backdrop (axes were black on black).
• Section highlighted in the nav on posts; lightbox returns focus to its figure; footer at the bottom of short pages and aligned with the column; 44px menu button; search shows category names without hyphens; print styles (black on white, no header/TOC/share/footer).

Gates (post_conventions.yml unchanged)

• script/lint-posts.rb: removed rules that never fired in any of 113 historical _posts/ snapshots and are unlikely under the workflow (tag case across posts, cross-line inline-span state); one CODE_SPAN/strip_code helper; use_math folded into the body pass; no ENV["TZ"] side effect; date-only values compared as dates. New rule: ar5iv links — fails ![x](https://ar5iv.labs.arxiv.org//html/2310.06825/assets/x1.png), passes ![x](https://arxiv.org/html/2401.04088v1/x1.png).
• script/check-plugin-output.rb now also: translation-listing check (twins from front matter, so a disabled hook cannot hide them), and one id per page (a tag spelled two ways gives two tag sections with one id; replaces the lint's tag-case rule).
• script/validate-site.sh: dropped the redirect-target check (stubs always target post.url, which the plugin check verifies) and the feed author grep (the lint forbids author:).
• CI passes the Pages base path through env:.

Cleanup

head.html: always-true fb_image guard removed; site.author replaces the duplicate name key. README authoring section trimmed to the rules authors trip on; tech-doc's rule enumeration replaced by a pointer to the script and tests.

Gates: tests (169), post lint, strict build, htmlproofer (CI flags), validate-site, shellcheck, actionlint; axe-core 0 violations; MathJax 0 errors.