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

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

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>
Signed-off-by: TMHSDigital <[email protected]>

Author: TMHSDigital

.github/workflows/validate.yml

@@ -142,6 +142,9 @@ jobs:
       - name: Check falsifiers declare the budget they target
         run: python3 tests/check_falsifier_targets.py
 
+      - 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..."