Add HasQuorum() to RaftNode that counts peers with LastContact within
2 × ElectionTimeoutMaxMs and returns true only when self + current peers
reaches majority. ProposeAsync now throws InvalidOperationException with
"no quorum" when HasQuorum() returns false, preventing a partitioned
leader from diverging the log. Add 14 tests in RaftQuorumCheckTests.cs
covering single-node, 3-node, 5-node, boundary window, and heartbeat
restore scenarios. Update RaftHealthTests.LastContact_updates_on_successful_replication
to avoid triggering the new quorum guard.
- ProposeAddPeerAsync/ProposeRemovePeerAsync: single-change-at-a-time membership
changes through RAFT consensus (Go ref: raft.go:961-1019)
- RaftLog.Compact: removes entries up to given index for log compaction
- CreateSnapshotCheckpointAsync: creates snapshot and compacts log in one operation
- DrainAndReplaySnapshotAsync: drains commit queue, installs snapshot, resets indices
- Pre-vote protocol skipped (Go NATS doesn't implement it either)
- 23 new tests in RaftMembershipAndSnapshotTests
