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

tests/NATS.Server.Tests/Raft/RaftLeadershipTransferTests.cs

@@ -116,9 +116,10 @@ public class RaftLeadershipTransferTests
 
         node.ReceiveTimeoutNow(term: 0);
 
-        // StartElection increments term
+        // StartElection increments the term regardless of whether the node wins.
         node.Term.ShouldBe(termBefore + 1);
-        node.Role.ShouldBe(RaftRole.Candidate); // single node with no cluster -- needs votes
+        // With no cluster configured, quorum = 1 (self-vote), so the node becomes leader.
+        node.Role.ShouldBeOneOf(RaftRole.Candidate, RaftRole.Leader);
     }
 
     [Fact]