fix cluster stubs: deterministic fallback, payload forwarding, and handler guards

- Replace non-deterministic map range fallback with sorted node selection in PlaceActor and RebalanceShards
- Add Body field to MessagePayload to preserve message data during cross-node routing
- Forward actual message body in route_message handler instead of discarding it
- Add self-message guard to route_message handler to prevent loops
- Add nil guard for shardMap in handleRebalanceRequest
- Add self-message guard to handleRebalanceRequest in DistributedVM

cluster/distributed.go

@@ -151,8 +151,10 @@ func (dvm *DistributedVM) SendMessage(message RuntimeMessage) error {
 // routeMessageToNode sends a message to another node for delivery to the target actor
 func (dvm *DistributedVM) routeMessageToNode(actorID string, message RuntimeMessage) error {
 	hops := 0
+	var body map[string]interface{}
 	if mp, ok := message.(*MessagePayload); ok {
 		hops = mp.Hops
+		body = mp.Body
 	}
 	if hops >= MaxRouteHops {
 		dvm.cluster.logger.Printf("Dropping message for actor %s: exceeded max hops (%d)", actorID, MaxRouteHops)
@@ -167,6 +169,7 @@ func (dvm *DistributedVM) routeMessageToNode(actorID string, message RuntimeMess
 			TargetActorID: actorID,
 			Type:          message.GetType(),
 			Hops:          hops + 1,
+			Body:          body,
 		},
 		Timestamp: time.Now(),
 	}
@@ -219,8 +222,10 @@ func (dvm *DistributedVM) handleClusterMessage(msg *nats.Msg) {
 		dvm.localRuntime.LoadModel(&model)
 
 	case "route_message":
-		// Handle message routing from other nodes
-		// Re-marshal and unmarshal to convert map[string]interface{} to concrete type
+		if clusterMsg.From == dvm.nodeID {
+			return
+		}
+
 		payloadBytes, err := json.Marshal(clusterMsg.Payload)
 		if err != nil {
 			return
@@ -236,11 +241,16 @@ func (dvm *DistributedVM) handleClusterMessage(msg *nats.Msg) {
 		}
 
 		targetActor := message.TargetActorID
+		msg := &MessagePayload{
+			TargetActorID: targetActor,
+			Type:          message.Type,
+			Hops:          message.Hops,
+			Body:          message.Body,
+		}
 		if dvm.IsLocalActor(targetActor) {
-			dvm.localRuntime.SendMessage(&message)
+			dvm.localRuntime.SendMessage(msg)
 		} else {
-			// Relay to the correct node
-			dvm.routeMessageToNode(targetActor, &message)
+			dvm.routeMessageToNode(targetActor, msg)
 		}
 
 	case "rebalance":
@@ -260,6 +270,11 @@ func (dvm *DistributedVM) handleRebalanceRequest(msg ClusterMessage) {
 		return
 	}
 
+	if dvm.cluster.shardMap == nil {
+		dvm.cluster.logger.Printf("Shard map is nil, skipping rebalance")
+		return
+	}
+
 	payloadBytes, err := json.Marshal(msg.Payload)
 	if err != nil {
 		dvm.cluster.logger.Printf("Failed to marshal rebalance payload: %v", err)