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>

3 files changed, 40 insertions, 10 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 "*".
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:-}"
 
diff --git a/src/lib/metadata-cache.source.sh b/src/lib/metadata-cache.source.sh
index aa580b6..2a367c7 100644
--- a/src/lib/metadata-cache.source.sh
+++ b/src/lib/metadata-cache.source.sh
@@ -10,6 +10,22 @@
 # dependency, it does not affect availability. Behaviour and signatures are
 # unchanged by the move.
 
+# The volatile EXIF cache directory: ./cache/exif parallel to ./dist. Computed
+# once here (task pr0) from working_dir() (the parent of DIST_DIR) so the path is
+# defined in exactly ONE place. It was previously recomputed inline -- byte for
+# byte identically -- in both cached_photo_identify_output (read/write) and
+# clear_exif_cache (remove for --force/--clean); a single source for it keeps
+# those two in lockstep so the reader and the cleaner can never drift apart and
+# point at different directories. The path is deliberately a sibling of DIST_DIR
+# (working_dir()/cache/exif): the staging dir is a sibling of the final dist, so
+# this resolves to the working dir in both staging and direct contexts, lives
+# outside dist (surviving a fresh/cleared dist and never deployed), and lets an
+# unchanged photo skip the slow `identify -verbose` on every regenerate.
+# printf (no echo) keeps it safe under `set -euo pipefail`.
+exif_cache_dir() {
+    printf '%s\n' "$(working_dir)/cache/exif"
+}
+
 # Build the cache signature line ("<photo>:<size>:<mtime>") used to decide
 # whether a cache entry is still valid for the source file. Kept private to this
 # module alongside its only consumers, plus the stats test that pre-seeds caches.
@@ -49,12 +65,11 @@ cached_photo_identify_output() {
     local current_signature
     local identify_status
 
-    # Persist the EXIF cache in a volatile ./cache directory parallel to ./dist
-    # (the staging dir is a sibling of the final dist, so dirname "$DIST_DIR" is
-    # the working dir in both staging and direct contexts). Keeping it outside
-    # dist means it survives a fresh/cleared dist and is never deployed, so an
-    # unchanged photo skips the slow `identify -verbose` on every regenerate.
-    cache_dir="$(dirname "$DIST_DIR")/cache/exif"
+    # Resolve the volatile EXIF cache dir via the shared exif_cache_dir() helper
+    # (see its definition above for why it sits parallel to ./dist and survives a
+    # cleared dist) so the reader/writer here and clear_exif_cache below always
+    # agree on the same directory.
+    cache_dir="$(exif_cache_dir)"
     cache_file="$cache_dir/$photo.txt"
     current_signature=$(photo_cache_signature "$photo" "$photo_path")
 
@@ -151,7 +166,7 @@ photo_exif_values_to() {
 # lifecycle counterpart of cached_photo_identify_output above, so the cache's
 # creation and destruction now live in the same module.
 clear_exif_cache() {
-    local -r cache_dir="$(dirname "$DIST_DIR")/cache/exif"
+    local -r cache_dir="$(exif_cache_dir)"
 
     log_verbose "Force generation; clearing EXIF cache $cache_dir"
     rm -rf "$cache_dir"