fix(gallery): describe each still in its alt text instead of the API it teaches

[TMHSDigital]

Summary

Closes #213. Every gallery image's alt described the API an entry teaches, not the picture: "{name} — {first sentence of teaches}", with 29 of 79 cut at 160 characters. Detail-page, zoom and landing images said only "{name} render".

Type

• feat
• fix — accessibility and content
• docs / chore / ci / refactor

Changes

• alt on all 79 entries in examples/gallery.json and showcase/gallery.json. Each is one sentence, written by looking at the committed hero; the longest is 135 characters. Examples:
  • light-link-studio: "Two spheres on pedestals: a brightly lit orange sphere labelled LINKED and a dark grey sphere labelled UNLINKED."
  • lod-decimate-chain: "Three red and white rockets labelled LOD0, LOD1 and LOD2, each more coarsely faceted than the last."
  • text-version-stamp stays version-neutral: "…the Blender version number in large gold 3D characters."
• build_gallery.py uses it for the card image, detail hero, zoom image and zoom button (aria-label="View full size: …"). New check_alts() fails the build on a missing alt or one over 200 characters. The card_alt / first_sentence / assert_alts_survive_dotted_paths helpers are removed; their only job was deriving the old alt.
• Landing page (template.html.j2): both card grids use ex.alt, falling back to the old text.
• Docs: CLAUDE.md (the one example-wiring sentence), CONTRIBUTING.md step 3 and docs/new-example-prompt.md say new entries need an alt.

Evidence

• live-run-proven (local, no Blender):
  • Regenerated gallery: 79 card alts, 0 ending in "…". stone-well detail hero and zoom both read "A round stone well of grey blocks under a four-post wooden roof, with a bucket hanging inside."
  • Landing page: all 14 card alts use alt; none end in " render".
  • Falsified: deleting campfire's alt makes build_gallery.py exit 1 with gallery entry 'campfire' has no 'alt': add one sentence describing what its still shows. Restored.
  • JSON diff: pure insertions (79 lines); both files round-trip byte-identically through json.
• inspection-only: alts describe the committed stills. For the 25 heroes Committed hero stills do not match what the scripts render #200 found drifted, I kept the wording general enough to hold after a re-render (e.g. treasure-chest "An open wooden treasure chest with a vaulted lid and dark iron bands").

Merge note

This touches build_gallery.py and template.html.j2, as does #220 (fonts and og card), but in different hunks. Whichever merges second gets main merged in and the gallery regenerated, which the new Validate drift check enforces.

Checklist

• Explicit paths only. The CLAUDE.md edit is deliberate and one line.
• Counts and manifest unchanged.
• The new check was falsified once (above).
• DCO Signed-off-by: present.
• No credentials, emails, or local paths.

🤖 Generated with Claude Code

[TMHSDigital]

Merged main (with #219 and #220) into this branch. The one conflict was in scripts/build_gallery.py constants: kept #220's OG_CARD_* and this PR's ALT_MAX, and dropped the retired _ALT_CAP. With real alts available, detail pages' og:image:alt / twitter:image:alt now use the entry's alt instead of ": the example's own headless render". Gallery regenerated in the merge commit.