pn0 decouple stats from album internals

Stats reached directly into album-module internals: the private global
ALBUM_VIEW_PAGE_BY_PHOTO and the EXIF cache reader
cached_photo_identify_output. Introduce a clean boundary, behaviour and
generated HTML byte-identical.

- Promote the EXIF identify cache primitive (cached_photo_identify_output
  plus its private helpers photo_cache_signature and
  print_cached_photo_identify_output) out of album-metadata.source.sh into
  a new shared src/lib/metadata-cache.source.sh, sourced before both album
  and stats (right after metadata-label in LIB_SOURCES). It is a low-level
  metadata primitive used by both consumers, so it no longer belongs to
  album internals. Signature/behaviour unchanged.
- Add album_view_page_for_photo accessor in album-render.source.sh as the
  documented public API; keep ALBUM_VIEW_PAGE_BY_PHOTO as the album's
  private backing store. stats-filter-album.source.sh now calls the
  accessor instead of indexing the global, so a change to album page
  naming/caching stays contained in the album module.
- Add test_album_stats_decoupling_boundary asserting the accessor returns
  the backing-store value and that the assembled bin/shuriken keeps the
  cache primitive in the shared module and no longer indexes the global
  from the stats filter section. Existing stats/album tests unchanged.

Verified: 3-image fixture (STATS_PAGE=yes, fixed seed) diff -r of stashed
original vs new build is byte-identical across all 47 dist files (only the
inherent generated_at timestamp normalized). just test, just shellcheck,
just check-generated and git diff --check all pass.

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

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