en0: make stats categories self-registering via STATS_CATEGORIES

Adding an EXIF stats category previously required editing four places: the
reset function, a new _stats_record_*, the body builder, and a new
_stats_render_* section. Introduce a STATS_CATEGORIES registry (the single
source of truth) and make the generic code iterate it instead.

- STATS_CATEGORIES: ordered, pipe-delimited specs
  (count_array|prefix|heading|render_kind), declared -gra so it survives a
  function-scoped source. The array order IS the overview display order.
- STATS_CATEGORY_BUCKETS: tab-delimited bucket ladders for the 'ordered'
  histogram kinds (apertures wide->narrow, etc.).
- STATS_RECORD_FUNCTIONS: the per-photo recorder dispatch list.

Collapsed touch-points:
- reset_photo_exif_stats clears each registry count array via
  _stats_category_arrays.
- accumulate_photo_stats dispatches recorders from STATS_RECORD_FUNCTIONS.
- _stats_build_body iterates STATS_CATEGORIES, dispatching each spec through
  _stats_render_category (camera/ranked/ordered/month kinds).
- _stats_render_ordered_section reads its ladder from STATS_CATEGORY_BUCKETS;
  the camera leaderboard is now ranked + a 'stats-leaderboard' list_class.

Adding a category is now: append one STATS_CATEGORIES entry (plus a
STATS_CATEGORY_BUCKETS row for an ordered ladder) and have a record function
tally into its array. No edits to reset, the body builder, or a per-category
render branch.

Behaviour-preserving: category order, bucket order, headings, counts and
links are unchanged. Verified byte-identical by diffing the stats/ output of
the pre-change binary against the new one over the same fixture album.

Added test_stats_categories_registry_is_single_source_of_truth (registered in
main()) asserting reset, the body builder and the bucket ladders all derive
from the registry; it fails if a category is added in only one place.

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

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