Comprehensive unit tests for shard management functionality:
- GetShard returns correct shard for actor IDs consistently
- GetShardNodes returns nodes responsible for each shard
- AssignShard correctly updates shard assignments
- PlaceActor returns valid nodes from available set
- Shard assignment handles node failures gracefully
- Replication factor is properly tracked
Includes tests for edge cases (empty shards, nil registry, single node)
and benchmark tests for GetShard, AssignShard, and PlaceActor.
Closes#5
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
