feat(raft): add membership proposals, snapshot checkpoints, and log compaction (B4+B5+B6)

- 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

src/NATS.Server/JetStream/Cluster/ClusterAssignmentTypes.cs

@@ -0,0 +1,49 @@
+namespace NATS.Server.JetStream.Cluster;
+
+/// <summary>
+/// RAFT group describing which peers own a replicated asset (stream or consumer).
+/// Go reference: jetstream_cluster.go:154-163 raftGroup struct.
+/// </summary>
+public sealed class RaftGroup
+{
+    public required string Name { get; init; }
+    public List<string> Peers { get; init; } = [];
+    public string StorageType { get; set; } = "file";
+    public string Cluster { get; set; } = string.Empty;
+    public string Preferred { get; set; } = string.Empty;
+
+    public int QuorumSize => (Peers.Count / 2) + 1;
+    public bool HasQuorum(int ackCount) => ackCount >= QuorumSize;
+}
+
+/// <summary>
+/// Assignment of a stream to a RAFT group of peers.
+/// Go reference: jetstream_cluster.go:166-184 streamAssignment struct.
+/// </summary>
+public sealed class StreamAssignment
+{
+    public required string StreamName { get; init; }
+    public required RaftGroup Group { get; init; }
+    public DateTime Created { get; init; } = DateTime.UtcNow;
+    public string ConfigJson { get; set; } = "{}";
+    public string SyncSubject { get; set; } = string.Empty;
+    public bool Responded { get; set; }
+    public bool Recovering { get; set; }
+    public bool Reassigning { get; set; }
+    public Dictionary<string, ConsumerAssignment> Consumers { get; } = new(StringComparer.Ordinal);
+}
+
+/// <summary>
+/// Assignment of a consumer to a RAFT group within a stream's cluster.
+/// Go reference: jetstream_cluster.go:250-266 consumerAssignment struct.
+/// </summary>
+public sealed class ConsumerAssignment
+{
+    public required string ConsumerName { get; init; }
+    public required string StreamName { get; init; }
+    public required RaftGroup Group { get; init; }
+    public DateTime Created { get; init; } = DateTime.UtcNow;
+    public string ConfigJson { get; set; } = "{}";
+    public bool Responded { get; set; }
+    public bool Recovering { get; set; }
+}