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/RaftWireFormat.cs

@@ -356,6 +356,93 @@ public readonly record struct RaftAppendEntryResponseWire(
     }
 }
 
+/// <summary>
+/// Binary wire encoding of a RAFT Pre-Vote request.
+/// Same layout as VoteRequest (32 bytes) — Go uses same encoding for pre-vote.
+/// The pre-vote round does NOT increment the term; it tests whether a candidate
+/// would win an election before disrupting the cluster.
+/// Go reference: raft.go:1600-1700 (pre-vote logic)
+/// </summary>
+public readonly record struct RaftPreVoteRequestWire(
+    ulong Term,
+    ulong LastTerm,
+    ulong LastIndex,
+    string CandidateId)
+{
+    /// <summary>
+    /// Encodes this PreVoteRequest to a 32-byte little-endian buffer.
+    /// Same layout as VoteRequest.
+    /// </summary>
+    public byte[] Encode()
+    {
+        var buf = new byte[RaftWireConstants.VoteRequestLen];
+        BinaryPrimitives.WriteUInt64LittleEndian(buf.AsSpan(0), Term);
+        BinaryPrimitives.WriteUInt64LittleEndian(buf.AsSpan(8), LastTerm);
+        BinaryPrimitives.WriteUInt64LittleEndian(buf.AsSpan(16), LastIndex);
+        RaftWireHelpers.WriteId(buf.AsSpan(24), CandidateId);
+        return buf;
+    }
+
+    /// <summary>
+    /// Decodes a PreVoteRequest from a span. Throws <see cref="ArgumentException"/>
+    /// if the span is not exactly 32 bytes.
+    /// </summary>
+    public static RaftPreVoteRequestWire Decode(ReadOnlySpan<byte> msg)
+    {
+        if (msg.Length != RaftWireConstants.VoteRequestLen)
+            throw new ArgumentException(
+                $"PreVoteRequest requires exactly {RaftWireConstants.VoteRequestLen} bytes, got {msg.Length}.",
+                nameof(msg));
+
+        return new RaftPreVoteRequestWire(
+            Term: BinaryPrimitives.ReadUInt64LittleEndian(msg[0..]),
+            LastTerm: BinaryPrimitives.ReadUInt64LittleEndian(msg[8..]),
+            LastIndex: BinaryPrimitives.ReadUInt64LittleEndian(msg[16..]),
+            CandidateId: RaftWireHelpers.ReadId(msg[24..]));
+    }
+}
+
+/// <summary>
+/// Binary wire encoding of a RAFT Pre-Vote response.
+/// Same layout as VoteResponse (17 bytes) with Empty always false.
+/// Go reference: raft.go:1600-1700 (pre-vote logic)
+/// </summary>
+public readonly record struct RaftPreVoteResponseWire(
+    ulong Term,
+    string PeerId,
+    bool Granted)
+{
+    /// <summary>
+    /// Encodes this PreVoteResponse to a 17-byte buffer.
+    /// Same layout as VoteResponse with Empty flag always false.
+    /// </summary>
+    public byte[] Encode()
+    {
+        var buf = new byte[RaftWireConstants.VoteResponseLen];
+        BinaryPrimitives.WriteUInt64LittleEndian(buf.AsSpan(0), Term);
+        RaftWireHelpers.WriteId(buf.AsSpan(8), PeerId);
+        buf[16] = Granted ? (byte)1 : (byte)0;
+        return buf;
+    }
+
+    /// <summary>
+    /// Decodes a PreVoteResponse from a span. Throws <see cref="ArgumentException"/>
+    /// if the span is not exactly 17 bytes.
+    /// </summary>
+    public static RaftPreVoteResponseWire Decode(ReadOnlySpan<byte> msg)
+    {
+        if (msg.Length != RaftWireConstants.VoteResponseLen)
+            throw new ArgumentException(
+                $"PreVoteResponse requires exactly {RaftWireConstants.VoteResponseLen} bytes, got {msg.Length}.",
+                nameof(msg));
+
+        return new RaftPreVoteResponseWire(
+            Term: BinaryPrimitives.ReadUInt64LittleEndian(msg[0..]),
+            PeerId: RaftWireHelpers.ReadId(msg[8..]),
+            Granted: (msg[16] & 1) != 0);
+    }
+}
+
 /// <summary>
 /// Shared encoding helpers for all RAFT wire format types.
 /// </summary>