Extract shared list_photos/pick_random_photo helper (br0)

Collapse the duplicated "find photos, error on empty, random_index by
ctx, return one" pattern into a single photo-list.source.sh exposing
list_photos <dir> and pick_random_photo <dir> <ctx>. Consumers in
album-photo-select, album-render, image-pipeline and stats-render now
call the shared helpers; _stats_load_background_photos keeps its caching
via a cached list_photos call rather than re-listing per filter page.
Per-call-site random_index ctx strings are preserved, so selection
semantics are unchanged. Sourced before its consumers in LIB_SOURCES
(Justfile + src/shuriken.sh); bin/shuriken regenerated.

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

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