feat: implement cross-node event broadcasting with NATSEventBus

- Add UpdateVersionCache method to JetStreamEventStore for cache synchronization
- Add SubscribeToEventStored convenience helper to NATSEventBus
- Create integration tests for cross-node broadcasting scenarios
- Add example demonstrating NATSEventBus + JetStreamEventStore integration
- Tests verify:
  - Single-node broadcasting works
  - Multi-node event flow works
  - Version cache consistency across nodes
  - Namespace isolation maintained
  - EventStored subscription works correctly

store/jetstream.go

@@ -558,5 +558,18 @@ func sanitizeSubject(s string) string {
 	return s
 }
 
+// UpdateVersionCache updates the version cache for a specific actor.
+// This is used when receiving events from other nodes via NATS to keep
+// the version cache consistent across cluster nodes.
+func (jes *JetStreamEventStore) UpdateVersionCache(actorID string, version int64) {
+	jes.mu.Lock()
+	defer jes.mu.Unlock()
+
+	// Only update if the new version is greater than cached version
+	if currentVersion, ok := jes.versions[actorID]; !ok || version > currentVersion {
+		jes.versions[actorID] = version
+	}
+}
+
 // Compile-time check that JetStreamEventStore implements EventStoreWithErrors
 var _ aether.EventStoreWithErrors = (*JetStreamEventStore)(nil)