implement cluster stubs: cross-node routing, rebalancing, and actor migration

- Fix ConsistentHashPlacement.PlaceActor() to use consistent hash ring
- Implement ConsistentHashPlacement.RebalanceShards() to redistribute shards
- Implement ClusterManager.handleRebalanceRequest() with actual rebalancing
- Implement ClusterManager.handleMigrationRequest() for actor state transfer
- Implement ClusterManager.triggerShardRebalancing() to compute and broadcast
- Implement DistributedVM.SendMessage() with cross-node NATS routing
- Implement DistributedVM.handleRebalanceRequest() to update shard map
- Fix route_message handler to check if actor is local before delivery
- Update ConsistentHashPlacement.RebalanceShards() test for new behavior
- Add handleShardMapUpdate() and broadcastShardMap() to ClusterManager

cluster/shard_test.go

@@ -650,7 +650,8 @@ func TestConsistentHashPlacement_RebalanceShards(t *testing.T) {
 	placement := &ConsistentHashPlacement{}
 	currentMap := &ShardMap{
 		Version: 1,
-		Shards:  map[int][]string{0: {"node-1"}},
+		Shards:  map[int][]string{0: {"node-1"}, 1: {"node-1"}, 2: {"node-2"}},
+		Nodes:   map[string]NodeInfo{},
 	}
 	nodes := map[string]*NodeInfo{
 		"node-1": {ID: "node-1"},
@@ -662,9 +663,24 @@ func TestConsistentHashPlacement_RebalanceShards(t *testing.T) {
 	if err != nil {
 		t.Errorf("unexpected error: %v", err)
 	}
-	// Current implementation returns unchanged map
-	if result != currentMap {
-		t.Error("expected same map returned (simplified implementation)")
+	if result == nil {
+		t.Fatal("rebalance returned nil")
+	}
+	if result.Version != currentMap.Version+1 {
+		t.Errorf("expected version %d, got %d", currentMap.Version+1, result.Version)
+	}
+	if len(result.Shards) != len(currentMap.Shards) {
+		t.Errorf("expected %d shards, got %d", len(currentMap.Shards), len(result.Shards))
+	}
+	for shardID, shardNodes := range result.Shards {
+		if len(shardNodes) == 0 {
+			t.Errorf("shard %d has no nodes assigned", shardID)
+		}
+		for _, node := range shardNodes {
+			if _, exists := nodes[node]; !exists {
+				t.Errorf("shard %d assigned to unknown node %s", shardID, node)
+			}
+		}
 	}
 }