FV-6: Liveness properties + fairness constraints

- mvcc.tla: Add CommitProgress liveness (verified with WF_vars)
- raft/twopc/mvcc/gossip: Add Spec definitions with WF_vars(Next)
- gossip.tla: Fix LearnViaGossip strength-based filtering
  (prevents overwriting Dead/Suspect with weaker state)
- twopc.tla: Add DecideCommitAction/DecideAbortAction for SF fairness
- Update all .cfg files to use SPECIFICATION Spec
- Update CHANGELOG.md -> v1.1.0, VERSION -> 1.1.0, PLAN.md

formal-verification/mvcc.tla

@@ -177,4 +177,13 @@ TypeOk ==
   /\ readSet \in [1..MaxTxnId -> SUBSET Keys]
   /\ globalClock \in 1..(MaxTxnId + 1)
 
+\* Liveness properties
+
+\* Any transaction that writes something eventually commits or aborts.
+CommitProgress ==
+  \A t \in 1..MaxTxnId : writeSet[t] /= {} ~> txnState[t] \in {"Committed", "Aborted"}
+
+\* Specification with weak fairness.
+Spec == Init /\ [][Next]_vars /\ WF_vars(Next)
+
 =============================================================================