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/shard.go

@@ -195,10 +195,12 @@ func (chp *ConsistentHashPlacement) PlaceActor(actorID string, shardMap *ShardMa
 
 	node := ring.GetNode(actorID)
 	if node == "" {
+		sortedNodeIDs := make([]string, 0, len(nodes))
 		for nodeID := range nodes {
-			return nodeID, nil
+			sortedNodeIDs = append(sortedNodeIDs, nodeID)
 		}
-		return "", fmt.Errorf("failed to place actor")
+		sort.Strings(sortedNodeIDs)
+		return sortedNodeIDs[0], nil
 	}
 
 	return node, nil
@@ -231,10 +233,12 @@ func (chp *ConsistentHashPlacement) RebalanceShards(currentMap *ShardMap, nodes
 	for shardID := range currentMap.Shards {
 		primaryNode := ring.GetNode(fmt.Sprintf("shard-%d", shardID))
 		if primaryNode == "" {
+			sortedNodeIDs := make([]string, 0, len(nodes))
 			for nodeID := range nodes {
-				primaryNode = nodeID
-				break
+				sortedNodeIDs = append(sortedNodeIDs, nodeID)
 			}
+			sort.Strings(sortedNodeIDs)
+			primaryNode = sortedNodeIDs[0]
 		}
 
 		var replicaNodes []string