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/models/twopc.cfg

@@ -3,8 +3,7 @@ CONSTANTS
   Nil = Nil
   MaxTxnId = 3
 
-INIT Init
-NEXT Next
+SPECIFICATION Spec
 
 CHECK_DEADLOCK FALSE
 
@@ -16,3 +15,4 @@ INVARIANTS
   NoDecideWithoutConsensus
   ParticipantStateValid
   RecoveryConsistency
+