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, 14 insertions, 0 deletions

diff --git a/src/lib/config.source.sh b/src/lib/config.source.sh
index caa2fe3..c676899 100644
--- a/src/lib/config.source.sh
+++ b/src/lib/config.source.sh
@@ -11,6 +11,20 @@ existing_parent_dir() {
     printf '%s\n' "$existing_parent"
 }
 
+# The generation "working" directory: the parent of DIST_DIR. Several DIST_DIR
+# derived paths hang off this single parent -- the staging/backup sibling dirs
+# (config.staging, action) and the volatile EXIF cache (metadata-cache) all live
+# beside the final dist. Centralised here (task pr0), next to existing_parent_dir
+# (the other DIST_DIR-parent resolver), so the plain `dirname "$DIST_DIR"` is
+# computed in exactly one place instead of being recomputed at each call site.
+# Plain dirname (NOT existing_parent_dir): callers that previously inlined
+# `dirname "$DIST_DIR"` get a byte-identical result, including any trailing-slash
+# or relative-vs-absolute behaviour dirname already produced. printf (no echo) so
+# it stays safe under `set -euo pipefail`.
+working_dir() {
+    printf '%s\n' "$(dirname "$DIST_DIR")"
+}
+
 resolve_config_file() {
     local -r config_file="${1:-}"