fix cluster: hashRing stale after rebalance, GetShardMap empty, loops, and migration no-op

- GetShardMap: copy Shards and Nodes map contents before returning
- hashRing: rebuild from shardMap.Nodes after every rebalance/shard_map update
- DistributedVM.handleRebalanceRequest: add leader check and self-broadcast guard
- route_message: add hop-count (MaxRouteHops=10) to prevent infinite loops
- handleMigrationRequest: broadcast migration updates instead of setting local copy

cluster/types.go

@@ -191,11 +191,15 @@ func (m *ModelPayload) GetID() string { return m.ID }
 // GetName implements RuntimeModel
 func (m *ModelPayload) GetName() string { return m.Name }
 
+// MaxRouteHops is the maximum number of hops a routed message can take before being dropped
+const MaxRouteHops = 10
+
 // MessagePayload is a concrete type for JSON-unmarshaling RuntimeMessage payloads.
 // Use this when receiving message data over the network.
 type MessagePayload struct {
 	TargetActorID string `json:"targetActorId"`
 	Type          string `json:"type"`
+	Hops          int    `json:"hops,omitempty"`
 }
 
 // GetTargetActorID implements RuntimeMessage