Extends TwoNodeClusterHarness with three lifecycle primitives:
- StopNodeBAsync() — graceful CoordinatedShutdown (Cluster.Leave)
- RestartNodeBAsync() — rebuild node B on same Akka port + same in-memory DB
- WaitForClusterSizeAsync(n) — converge assertion helper
Adds three failover scenario tests:
- Stopping node B shrinks cluster to 1 Up member
- Restarted node B rejoins on the same Akka port
- Deployment started with B down seals with a single NodeDeploymentState
(validates ConfigPublishCoordinator.DiscoverDriverNodes snapshots
membership at dispatch time)
Closes follow-up F22. Integration test count: 6 → 9 (+3).
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