rename: failover → fallback throughout codebase — this system is Fallback, state is FALLBACK; preserve conventional failover comparisons

Docker_Essentials/docker_update.sh

@@ -43,7 +43,7 @@
 #
 # Version Ownership
 #   Fallback containers are excluded from remainder mode. This server only runs
-#   them during a failover. The remote server owns their version — if remainder
+#   them during a fallback. The remote server owns their version — if remainder
 #   updates them independently and a handback occurs, the remote's older image
 #   may not handle data written by the newer version.
 #
@@ -72,7 +72,7 @@
 #
 # Fallback Exclusion
 #   Remainder mode excludes containers owned by the remote server's update cycle
-#   to prevent version divergence across the failover boundary.
+#   to prevent version divergence across the fallback boundary.
 #
 # Running-Only Filter
 #   Stopped containers excluded from remainder mode — intentionally down.
@@ -179,7 +179,7 @@ if [[ "$REMAINDER_MODE" == true ]]; then
     unset _weekly_str _weekly_arr
 
     # Fallback coverage containers — owned by the remote server's update cycle.
-    # This server runs them during failover but should never update them independently.
+    # This server runs them during fallback but should never update them independently.
     # Updating them here risks version divergence: if remote's writeback after handback
     # encounters data written by a newer version, it may not handle it correctly.
     for _tier in 1 2 3 4; do