Add integration tests asserting every generated tile/output shape (xr0)

Closes the test-coverage blind spot the gauge-bar regression exposed: the
rendered grid tile types had zero content assertions. Adds three
deterministic integration tests (seeded layouts):

- test_generate_preview_grid_emits_every_tile_shape: single thumbnail
  (class='thumb'), 2x2 feature (class='feature' + a.feature span CSS),
  subdivided (<div class='tile'> + subthumb), the thumbs-grid container,
  all four breakpoint column rules (repeat 2/3/4/6), the view-page body
  and prev/next nav arrows. Pinned 30 photos, RANDOM_SEED, FEATURE=60,
  SUBDIVIDE=70, SHUFFLE=no for reproducibility.
- test_generate_short_final_page_emits_fill_row_tiles: the short-final-page
  full-width banner (class='fill-row' + grid-column:1/-1), absent on a full
  page. Pinned 13 photos, MAXPREVIEWS=10.
- test_generate_splash_page_renders_enter_link_and_photo: splash <main>,
  title, splash photo, Enter-album link.

Principle: every generated output format now has >=1 assertion on its
meaningful content, not just existence or render-equality. No src changes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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