or0: encapsulate STATS_* maps behind aggregator accessors

The stats reader modules (stats-render.source.sh, stats-filter-album.source.sh)
indexed stats-aggregate.source.sh's private STATS_* associative arrays directly,
so a key-convention change in the aggregator would silently break both readers.

Add a read API owned by stats-aggregate.source.sh (the data owner), mirroring
album-render's ALBUM_VIEW_PAGE_BY_PHOTO / album_view_page_for_photo split:

  stats_total_photos            - STATS_TOTALS[photos] denominator
  stats_filter_pagebase         - (prefix,label) -> pagebase, hiding the
                                  STATS_FILTER_KEYSEP catkey encoding
  stats_filter_title            - STATS_FILTER_TITLE[pagebase]
  stats_filter_photos           - STATS_FILTER_PHOTOS[pagebase] list
  stats_filter_count            - number of filter mini-albums
  stats_filter_pagebases        - pagebases, LC_ALL=C-sorted (order owned here)
  stats_category_count/size/max/keys_by_count_desc
                                - per-category count-array reads

Route every cross-module STATS_* read through these accessors. The render
module's _stats_max_count / _stats_keys_by_count_desc were duplicates of the new
stats_category_max / stats_category_keys_by_count_desc, so they are removed.

Behavior preserved exactly: missing-key semantics, iteration order (ordered
ladders, calendar months, count-desc with LC_ALL=C tie-break, sorted pagebase
enqueue) and generated HTML are byte-identical. Header docs updated to describe
the accessor boundary. The arrays stay the backing store; only the cross-module
READ path is encapsulated.

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

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