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/arr_sync.sh

@@ -10,6 +10,12 @@
 # rsync in the weekly sync window: once arrs agree on what to track, rsync
 # spreads the actual files.
 #
+# The LOCAL side of each sync (_local_library()) is cache-first (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. The REMOTE side (_remote_library()) is
+# unaffected — that cache is per-host by design, so a remote node's library is always
+# fetched live here.
+#
 # ==============================================================================================
 # OPERATIONAL MODEL
 # ==============================================================================================