# Issue: Implement Actor Migration Between Cluster Nodes

## Problem

When nodes join or leave the cluster, actors need to be migrated to maintain even distribution. Currently:

- `handleRebalanceRequest` in `cluster/manager.go:150` is empty
- `handleMigrationRequest` in `cluster/manager.go:167` is empty  
- `RebalanceShards` in `cluster/shard.go:211` returns unchanged map
- `SendMessage` in `cluster/distributed.go:139` ignores sharding

## Required Implementation

### 1. Rebalance Algorithm (cluster/shard.go)
Implement `ConsistentHashPlacement.RebalanceShards` to:
- Calculate new shard assignments based on active nodes
- Identify actors needing migration
- Generate migration plan with source/dest nodes

### 2. Migration Coordinator (cluster/manager.go)
Implement `handleRebalanceRequest` to:
- Accept migration plan from leader
- For each actor in plan:
  1. Pause incoming messages
  2. Capture actor state (replay events up to current version)
  3. Serialize state
  4. Send migration request to destination node
  5. Wait for ack
  6. Delete actor from current node
- Track migration status via `ActorMigration.Status`

### 3. Cross-Node Message Routing (cluster/distributed.go)
Implement proper routing in `SendMessage`:
- Use `GetActorNode(actorID)` to determine target node
- If remote: marshal message, send via NATS to target node
- If local: send to local runtime
- Route response back to caller if needed

## Suggested Approach

1. **Define message types** for actor migration requests/responses in `cluster/types.go`
2. **Implement state capture** - replay events to get current state
3. **Implement state restore** - deserialize and restore actor state
4. **Implement coordinator** - manage migration phases
5. **Add error handling** - handle failed migrations, retries, cleanup
6. **Add tests** - test migration with mock NATS

## Related Files

- `cluster/manager.go:150` - handleRebalanceRequest (empty)
- `cluster/manager.go:167` - handleMigrationRequest (empty)
- `cluster/shard.go:211` - RebalanceShards (stub)
- `cluster/distributed.go:139` - SendMessage (simplified)
- `cluster/types.go:108` - ActorMigration struct

## Acceptance Criteria

- [ ] `RebalanceShards` returns new shard map with actor assignments
- [ ] `handleRebalanceRequest` processes migration plan
- [ ] `handleMigrationRequest` accepts actor migrations
- [ ] `SendMessage` routes to correct node
- [ ] Actors can be migrated with state preserved
- [ ] Failed migrations are handled gracefully
- [ ] Integration test with multi-node cluster