fix(showcase): bring 19 previews to the 1200x675 spec and enforce hero/preview sizes

[TMHSDigital]

Summary

Closes #214. CLAUDE.md fixes preview webps at 1200×675 and heroes at 1280×720. All 51 example previews conformed, but 19 of 28 showcase previews shipped at the hero size, and nothing checked either size.

Type

• feat
• fix — content integrity plus a new check
• docs / chore / ci / refactor

Changes

• Downscaled 19 previews to 1200×675 (Lanczos, webp q90): stone-well, campfire, market-stall, street-lantern, terrain-scatter, fence-kit, watchtower, cart, park-bench, anvil, water-trough, hitching-post, hand-pump, signpost, chopping-block, wall-torch, wooden-ladder, crate-stack, stone-archway. Same 16:9 aspect ratio, so no re-render.
• New tests/check_gallery_images.py, wired into Validate. It asserts hero 1280×720 and preview 1200×675 for every entry in both gallery.json files. It uses only the standard library, parsing the WebP RIFF header (VP8 / VP8L / VP8X chunks), so it needs no Pillow on the runner.

Evidence

• live-run-proven (local, no Blender):
  • Falsified: before the downscale, the checker exits 1 with 19 errors, e.g. stone-archway: preview showcase/stone-archway/preview.webp is 1280x720, expected 1200x675. After it: gallery image sizes ok: 158 image(s) across both gallery.json files., exit 0.
  • Parser cross-check: webp_size() agrees with Pillow's Image.size on all 158 hero and preview files (0 disagreements).
  • Gallery unchanged: python scripts/build_gallery.py leaves docs/gallery/ untouched. The site does not serve previews.
• inspection-only: if Committed hero stills do not match what the scripts render #200 regenerates heroes, previews should be regenerated from the same render. This PR does not address drift.

Checklist

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

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