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/playback_aware_lidarr_discovery.sh

@@ -33,6 +33,11 @@
 #   8. Score candidates: affinity + breadth + popularity + quality
 #   9. Take top MAX_ADDS above threshold → add to Lidarr
 #
+# Step 7's "already in Lidarr" check reads the shared tracked-data cache via
+# arr_get_tracked_data() (cache-first, live fallback, 2026-07-17) instead of a live fetch —
+# this runs weekly right after arr_full_rescan.sh, so it's reading the genuine post-rescan
+# snapshot arr_full_rescan.sh just wrote.
+#
 # ==============================================================================================
 # SCORING MODEL
 # ==============================================================================================