fix(showcase): bring 19 previews to the 1200x675 spec and enforce hero/preview sizes (#217)

CLAUDE.md fixes preview webps at 1200x675 and heroes at 1280x720. All 51
example previews conformed, but 19 of 28 showcase previews shipped at the
hero size, and nothing checked either.

- Downscale the 19 (same 16:9 aspect, Lanczos, webp q90); no re-render.
- tests/check_gallery_images.py asserts both sizes for every gallery.json
  entry. Stdlib only: it parses the WebP RIFF header (VP8, VP8L, VP8X), so
  it runs on the bare Validate runner. Its parser agrees with Pillow on all
  158 images.

Closes #214

Signed-off-by: TMHSDigital <154358121+TMHSDigital@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: TMHSDigital

.github/workflows/validate.yml

@@ -156,6 +156,9 @@ jobs:
           fi
           echo "docs/gallery/ matches scripts/build_gallery.py output."
 
+      - name: Check gallery hero and preview sizes
+        run: python3 tests/check_gallery_images.py
+
       - name: Validate template Python syntax
         run: |
           echo "Checking template Python syntax..."