Fix mobile lightbox, print, code blocks; JPEG for two photos

[bits-bytes-nn]

Adversarial review loop (code/tests/CI/docs and UI/UX/visuals), 4 rounds; round 4: 0 MAJOR in both reviews.

Fixed on the live site

• Mobile lightbox: the caption covered the bottom of the figure (a heatmap's last rows, a diagram's bottom row). It now sits below the figure, the two centred together, the figure clear of the close button.
• Print: wide code, tables and display math were clipped at the page edge in 19 posts. Code wraps, tables and display math shrink; the cover band and the copy-button strip are gone.
• Copy button: translucent over the first code line on phones; now opaque, with the code starting below it.
• Dark mode code blocks: a band behind every line (the inline-code background); gone. Fences in a language Rouge does not know (the cedar blocks) get the same dark panel.
• Code text was 0.81em (two 0.9em rules compounding); now the intended 0.9em.

Gates and tests

• validate-site.sh compares description and title length in characters in any locale (it counted bytes under LC_ALL=C), without spawning a process per page.
• Tests that could not fail now can (math stripped before the tag scan); new tests for the type cover's precedence and lowercase tag words.

Assets

Two photographic Gemini figures as JPEG (2.6 MB → 552 KB, 2.0 MB → 272 KB). post_conventions.yml unchanged.

Gates: tests (173), post lint, strict build, htmlproofer (CI flags), validate-site (also under LC_ALL=C), shellcheck, actionlint; axe-core 0 violations.