feat: add leadership transfer via TimeoutNow RPC (Gap 8.4)

- Add RaftTimeoutNowWire: 16-byte wire type [8:term][8:leaderId] with
  Encode/Decode roundtrip, matching Go's sendTimeoutNow wire layout
- Add TimeoutNow(group) subject "$NRG.TN.{group}" to RaftSubjects
- Add SendTimeoutNowAsync to IRaftTransport; implement in both
  InMemoryRaftTransport (synchronous delivery) and NatsRaftTransport
  (publishes to $NRG.TN.{group})
- Add TransferLeadershipAsync(targetId, ct) to RaftNode: leader sends
  TimeoutNow RPC, blocks proposals via _transferInProgress flag, polls
  until target becomes leader or 2x election timeout elapses
- Add ReceiveTimeoutNow(term) to RaftNode: target immediately starts
  election bypassing pre-vote, updates term if sender's term is higher
- Block ProposeAsync with InvalidOperationException during transfer
- 15 tests in RaftLeadershipTransferTests covering wire roundtrip,
  ReceiveTimeoutNow behaviour, proposal blocking, target leadership,
  timeout on unreachable peer, and transfer flag lifecycle

src/NATS.Server/Raft/NatsRaftTransport.cs

@@ -198,4 +198,23 @@ public sealed class NatsRaftTransport : IRaftTransport
         var payload = System.Text.Encoding.UTF8.GetBytes(peer);
         _publish(removePeerSubject, null, payload);
     }
+
+    /// <summary>
+    /// Sends a TimeoutNow RPC to the target follower, asking it to immediately
+    /// start an election to facilitate leadership transfer.
+    ///
+    /// Publishes a <see cref="RaftTimeoutNowWire"/>-encoded payload to
+    /// <c>$NRG.TN.{group}</c>. The target node's message handler decodes
+    /// it and calls <see cref="RaftNode.ReceiveTimeoutNow"/>.
+    ///
+    /// Go reference: raft.go sendTimeoutNow
+    /// </summary>
+    public Task SendTimeoutNowAsync(string leaderId, string targetId, ulong term, CancellationToken ct)
+    {
+        _ = targetId;
+        var subject = RaftSubjects.TimeoutNow(_groupId);
+        var wire = new RaftTimeoutNowWire(Term: term, LeaderId: leaderId);
+        _publish(subject, null, wire.Encode());
+        return Task.CompletedTask;
+    }
 }

src/NATS.Server/Raft/RaftSubjects.cs

@@ -50,4 +50,12 @@ public static class RaftSubjects
     /// Go: server/raft.go:2168 — raftCatchupReply = "$NRG.CR.%s"
     /// </summary>
     public static string CatchupReply(string id) => $"$NRG.CR.{id}";
+
+    /// <summary>
+    /// TimeoutNow subject for the given RAFT group.
+    /// Sent by a leader to a target follower to trigger immediate election
+    /// for leadership transfer. Mirrors Go's sendTimeoutNow pattern.
+    /// Go reference: raft.go sendTimeoutNow
+    /// </summary>
+    public static string TimeoutNow(string group) => $"$NRG.TN.{group}";
 }

src/NATS.Server/Raft/RaftTransport.cs

@@ -5,6 +5,13 @@ public interface IRaftTransport
     Task<IReadOnlyList<AppendResult>> AppendEntriesAsync(string leaderId, IReadOnlyList<string> followerIds, RaftLogEntry entry, CancellationToken ct);
     Task<VoteResponse> RequestVoteAsync(string candidateId, string voterId, VoteRequest request, CancellationToken ct);
     Task InstallSnapshotAsync(string leaderId, string followerId, RaftSnapshot snapshot, CancellationToken ct);
+
+    /// <summary>
+    /// Sends a TimeoutNow RPC to the target follower, asking it to immediately start
+    /// an election and bypass its election timer. Used for leadership transfer.
+    /// Go reference: raft.go sendTimeoutNow
+    /// </summary>
+    Task SendTimeoutNowAsync(string leaderId, string targetId, ulong term, CancellationToken ct);
 }
 
 public sealed class InMemoryRaftTransport : IRaftTransport
@@ -61,4 +68,18 @@ public sealed class InMemoryRaftTransport : IRaftTransport
 
         await Task.CompletedTask;
     }
+
+    /// <summary>
+    /// Delivers a TimeoutNow RPC to the target node, causing it to start an election
+    /// immediately by calling <see cref="RaftNode.ReceiveTimeoutNow"/>.
+    /// If the target is not registered (simulating an unreachable peer), does nothing.
+    /// Go reference: raft.go sendTimeoutNow / processTimeoutNow
+    /// </summary>
+    public Task SendTimeoutNowAsync(string leaderId, string targetId, ulong term, CancellationToken ct)
+    {
+        _ = leaderId;
+        if (_nodes.TryGetValue(targetId, out var node))
+            node.ReceiveTimeoutNow(term);
+        return Task.CompletedTask;
+    }
 }