Add tests/tla_faithfulness.nim verifying Nim code against TLA+ invariants:
- Raft: ElectionSafety, LogMatching, CommittedIndexValid
- MVCC: NoDirtyReads, CommittedMustStart
- 2PC: Atomicity, RecoveryConsistency
Documented gap: Nim MVCC allows multiple committed versions per key,
while TLA+ spec enforces first-committer-wins (WriteWriteConflict).
This test ensures future changes to Nim code maintain alignment with specs.
Update PLAN.md (FV-14 done), CHANGELOG.md, test_all.nim includes new suite.
- Add heartbeatReceived variable tracking last term with heartbeat
- Add Heartbeat action: leader sends keep-alive; recipient steps down on higher term
- Add HeartbeatTimeout: follower starts election if no heartbeat in current term
- Add LeaderLeaseExpired: leader steps down if it loses quorum connectivity
- Add LeaderHasSelfHeartbeat invariant: every leader must have valid heartbeat
- raft.tla now models realistic partition behavior (38M states verified)
- Update CHANGELOG -> v1.2.0, VERSION -> 1.2.0, PLAN.md, README.md
dimgigov