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;
+    }
 }