Add OPERATIONAL SAFEGUARDS headers, skip prefill during active rescans

arr_full_rescan.sh, arr_cache_prefill.sh, and arr_rescan_monitor.sh were
missing the standard SAFEGUARDS header section other Arrs_Stack/Tools
scripts have. Also: arr_cache_prefill.sh now checks for an active rescan
before fetching, instead of doing a live fetch that arr_cache_write()
would just refuse to persist anyway -- avoids wasted API calls every
30min during a long rescan. arr_rescan_monitor.sh was also missing an
actual root check despite writing cache files; added it to match
convention rather than just document a safeguard that wasn't there.

Arrs_Stack/arr_full_rescan.sh

@@ -20,6 +20,24 @@
 # and running them concurrently would just contend for the same disk I/O for no benefit.
 #
 # ==============================================================================================
+# OPERATIONAL SAFEGUARDS
+# ==============================================================================================
+#
+#   Root required       — required for consistency across Arrs_Stack/, no direct fs writes here
+#   acquire_lock "wait" — waits for a prior run to finish rather than skipping or colliding;
+#                         a full-library rescan across three arrs can run long, worth queuing
+#                         behind rather than silently no-op'ing
+#   Reachability check  — check_api before touching an arr; unreachable → skip that arr only
+#   Active-rescan check — skips triggering a NEW rescan if one's already active on that arr
+#                         (manual trigger, another script) — never stacks a duplicate scan,
+#                         see Tools/arr_rescan_monitor.sh for catching up that arr's cache
+#                         once the pre-existing scan finishes instead of waiting for next week
+#   Sequential only     — never runs two arrs' rescans in parallel; each is a heavy full-disk
+#                         walk and concurrent walks would just contend for the same disk I/O
+#   Per-arr isolation   — one arr failing, timing out, or being skipped never blocks the others
+#   --dry-run mode      — reports which arrs would be rescanned, triggers nothing
+#
+# ==============================================================================================
 # CONFIGURATION
 # ==============================================================================================
 #