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/Raft/RaftMembership.cs

@@ -0,0 +1,49 @@
+namespace NATS.Server.Raft;
+
+/// <summary>
+/// Type of membership change operation.
+/// Go reference: raft.go:2500-2600 (ProposeAddPeer/RemovePeer)
+/// </summary>
+public enum RaftMembershipChangeType
+{
+    AddPeer,
+    RemovePeer,
+}
+
+/// <summary>
+/// Represents a pending RAFT membership change (add or remove peer).
+/// Serialized as "{Type}:{PeerId}" in log entry commands for wire compatibility.
+/// Go reference: raft.go:2500-2600 (membership change proposals)
+/// </summary>
+public readonly record struct RaftMembershipChange(RaftMembershipChangeType Type, string PeerId)
+{
+    /// <summary>
+    /// Encodes this membership change as a log entry command string.
+    /// Format: "AddPeer:node-id" or "RemovePeer:node-id"
+    /// </summary>
+    public string ToCommand() => $"{Type}:{PeerId}";
+
+    /// <summary>
+    /// Parses a log entry command string back into a membership change.
+    /// Returns null if the command is not a membership change.
+    /// </summary>
+    public static RaftMembershipChange? TryParse(string command)
+    {
+        var colonIndex = command.IndexOf(':');
+        if (colonIndex < 0)
+            return null;
+
+        var typePart = command[..colonIndex];
+        var peerPart = command[(colonIndex + 1)..];
+
+        if (string.IsNullOrEmpty(peerPart))
+            return null;
+
+        return typePart switch
+        {
+            nameof(RaftMembershipChangeType.AddPeer) => new RaftMembershipChange(RaftMembershipChangeType.AddPeer, peerPart),
+            nameof(RaftMembershipChangeType.RemovePeer) => new RaftMembershipChange(RaftMembershipChangeType.RemovePeer, peerPart),
+            _ => null,
+        };
+    }
+}