Update headers on today's arr-caching changes

Comment-only. Headers on the scripts touched during today's caching work
(cache-first fetches, write-through per-item cache, single-walk
consolidation, movieFile-embedded fix) still described pre-change
behavior. Also brought common.sh's top-level cache doc block current --
it was written for the single-consumer 2026-07-16 state and didn't
mention the tmpfs move, the write guard, or the 15+ consumers that now
go through it.

Arrs_Stack/lidarr_missing_art.sh

@@ -24,6 +24,14 @@
 # tags or renames media files. Only writes missing artwork files to existing
 # album/artist directories.
 #
+# Cache-first, both layers (2026-07-17). The artist list comes from the shared tracked-data
+# cache via arr_get_tracked_data() — fresh (kept warm every 30min by arr_cache_prefill.sh),
+# live fetch as fallback. The per-artist track-file walk used to build the album→directory
+# map now reads lidarr_cleanup.sh's write-through cache first (arr_get_cached_items() —
+# lidarr_cleanup.sh runs earlier in the same nightly window and already does this exact
+# walk for its own cleanup decisions), falling back to its own live per-artist walk only if
+# that cache is missing or from outside the current window.
+#
 # ==============================================================================================
 # DESIGN PRINCIPLES
 # ==============================================================================================