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

Fallback/README-Fallback.md

@@ -1,6 +1,6 @@
 # ━━━━━ FALLBACK ━━━━━
 
-Mutual automatic failover between two independent unRAID servers. When one goes down the
+Mutual automatic fallback between two independent unRAID servers. When one goes down the
 other starts its containers, cuts over DNS, and keeps users online. When it comes back
 everything hands back in the correct sequence — covering DDNS stops, containers stop, rsync
 writeback runs, containers start on the primary, primary DDNS starts last — so users hit the
@@ -45,7 +45,7 @@ The fix: stop containers before syncing. The outage window is only the rsync dur
 typically minutes. Clean static source at full bandwidth, predictable state every time.
 
 **No Way to Validate the System Before Needing It**  
-A failover system that has never been tested is not a failover system — it is a hope.  
+A fallback system that has never been tested is not a fallback system — it is a hope.  
 The fix: `fallback_test.sh` — a controlled simulation using an iptables DROP rule to make
 the remote appear unreachable, triggering the full sequence without taking anything offline.
 A safety trap removes the rule on any exit — crash, error, ctrl-c, or clean completion.