Centralize DIST_DIR-derived paths via working_dir/exif_cache_dir helpers

The EXIF cache dir ($(dirname "$DIST_DIR")/cache/exif) was recomputed inline,
byte for byte, in both cached_photo_identify_output (read/write) and
clear_exif_cache (--force/--clean removal). The plain parent of DIST_DIR
(dirname "$DIST_DIR") was likewise recomputed in metadata-cache and in
action.source.sh's staging-artifact cleanup.

Extract two helpers computed once from DIST_DIR:
  - working_dir() in config.source.sh (next to existing_parent_dir, the other
    DIST_DIR-parent resolver): plain `dirname "$DIST_DIR"`.
  - exif_cache_dir() in metadata-cache.source.sh (owns the EXIF cache):
    `working_dir()/cache/exif`.

Route cached_photo_identify_output, clear_exif_cache, and
clean_generation_staging_artifacts through them so the cache reader and the
cleaner can never drift to different directories. Paths are byte-identical to
the prior inline code (dirname semantics, cache/exif suffix, trailing-slash and
relative/absolute handling all preserved). Stale "recompute the cache dir"
comments removed; helpers document the path once. Add a unit test asserting the
helpers agree and resolve beside dist for several DIST_DIR shapes.

Broader DIST_DIR parameterization of leaf pipeline helpers was intentionally
left out of scope (only the duplicated path computation is centralized here).

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

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