feat: raft consensus for cluster configuration

[kacy]

Summary

• implements openraft integration for cluster configuration consensus
• adds TypeConfig for raft node/entry type configuration
• adds ClusterCommand enum for topology changes (add/remove node, assign slots, migrations)
• adds Storage struct implementing RaftStorage trait with in-memory log and state machine
• adds ClusterStateData and ClusterSnapshot for state serialization

Tested

• cargo test -p ember-cluster - 41 tests pass
• cargo test - 216 tests pass
• cargo clippy -p ember-cluster - no warnings

Design considerations

• uses single raft group for entire cluster (simpler than multi-raft per slot range)
• in-memory storage for now; persistence can be added later
• state machine tracks nodes, slots, and active migrations
• migration workflow split into BeginMigration/CompleteMigration commands