feat(monitoring+events): add connz filtering, event payloads, and message trace context (E12+E13+E14)

- Add ConnzHandler with sorting, filtering, pagination, CID lookup, and closed connection ring buffer
- Add full Go events.go parity types (ConnectEventMsg, DisconnectEventMsg, ServerStatsMsg, etc.)
- Add MessageTraceContext for per-message trace propagation with header parsing
- 74 new tests (17 ConnzFilter + 16 EventPayload + 41 MessageTraceContext)

src/NATS.Server/Events/InternalEventSystem.cs

@@ -159,6 +159,16 @@ public sealed class InternalEventSystem : IAsyncDisposable
                 Connections = _server.ClientCount,
                 TotalConnections = Interlocked.Read(ref _server.Stats.TotalConnections),
                 Subscriptions = SystemAccount.SubList.Count,
+                Sent = new DataStats
+                {
+                    Msgs = Interlocked.Read(ref _server.Stats.OutMsgs),
+                    Bytes = Interlocked.Read(ref _server.Stats.OutBytes),
+                },
+                Received = new DataStats
+                {
+                    Msgs = Interlocked.Read(ref _server.Stats.InMsgs),
+                    Bytes = Interlocked.Read(ref _server.Stats.InBytes),
+                },
                 InMsgs = Interlocked.Read(ref _server.Stats.InMsgs),
                 OutMsgs = Interlocked.Read(ref _server.Stats.OutMsgs),
                 InBytes = Interlocked.Read(ref _server.Stats.InBytes),