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>

1 files changed, 4 insertions, 3 deletions

diff --git a/src/lib/action.source.sh b/src/lib/action.source.sh
index c667519..a038fa7 100644
--- a/src/lib/action.source.sh
+++ b/src/lib/action.source.sh
@@ -179,14 +179,15 @@ log_configured_action() {
 # Safety: callers MUST run validate_clean_dist_dir first so a dangerous DIST_DIR
 # aborts before any deletion. We only match shuriken's own, basename-specific
 # staging/backup prefixes (never a loose ".shuriken.*" or arbitrary dotfiles),
-# derive the parent exactly as the staging code does (dirname "$DIST_DIR"), and
-# use nullglob so a missing match never expands to a literal pattern to rm.
+# derive the parent via working_dir() (the shared plain `dirname "$DIST_DIR"`,
+# exactly the parent the staging code stages into), and use nullglob so a missing
+# match never expands to a literal pattern to rm.
 clean_generation_staging_artifacts() {
     local final_base final_parent artifact
     local -a artifacts=()
 
     final_base=$(basename "$DIST_DIR")
-    final_parent=$(dirname "$DIST_DIR")
+    final_parent=$(working_dir)
 
     # nullglob: a non-matching glob expands to nothing rather than to the
     # literal pattern, so we never accidentally rm a path called "*".