feat: wire system event publishing for connect, disconnect, and shutdown

src/NATS.Server/Events/InternalEventSystem.cs

@@ -127,6 +127,11 @@ public sealed class InternalEventSystem : IAsyncDisposable
         return sub;
     }
 
+    /// <summary>
+    /// Returns the next monotonically increasing sequence number for event ordering.
+    /// </summary>
+    public ulong NextSequence() => Interlocked.Increment(ref _sequence);
+
     /// <summary>
     /// Enqueue an internal message for publishing through the send loop.
     /// </summary>