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/Monitoring/Connz.cs

@@ -218,6 +218,18 @@ public sealed class ConnzOptions
 
     public string MqttClient { get; set; } = "";
 
+    /// <summary>
+    /// When non-zero, returns only the connection with this CID.
+    /// Go reference: monitor.go ConnzOptions.CID.
+    /// </summary>
+    public ulong Cid { get; set; }
+
+    /// <summary>
+    /// Whether to include authorized user info.
+    /// Go reference: monitor.go ConnzOptions.Username.
+    /// </summary>
+    public bool Auth { get; set; }
+
     public int Offset { get; set; }
 
     public int Limit { get; set; } = 1024;