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_release_fixer.sh

@@ -34,6 +34,11 @@
 # RefreshArtist is batched — one per artist, even if multiple albums were fixed.
 # Lidarr handles the post-refresh rescan and import automatically.
 #
+# Cache-first artist list (2026-07-17) — 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. Everything below (per-album release lookups) still fetches live — that data
+# isn't part of what's cached.
+#
 # ==============================================================================================
 # DESIGN PRINCIPLES
 # ==============================================================================================