feat(cluster): add slot management and topology foundations

[kacy]

summary

implements the core building blocks for redis cluster compatibility:

• slots.rs: CRC16 hash function (redis-compatible), SlotMap for 16384 slot distribution, SlotRange for contiguous slot assignments, hash tag extraction for key co-location
• topology.rs: NodeId (uuid-based), ClusterNode with roles and health, ClusterState for full cluster view, MOVED redirect generation, CLUSTER INFO and CLUSTER NODES output formatting
• error.rs: ClusterError enum for MOVED, ASK, and other cluster errors
• protocol extensions: CLUSTER INFO/NODES/SLOTS/KEYSLOT/MYID commands and ASKING flag

this lays the foundation for gossip-based membership, raft consensus, and live slot migration.

what was tested

• unit tests for CRC16 implementation verified against known redis slot assignments
• unit tests for hash tag extraction ({tag} syntax)
• unit tests for SlotMap operations (assignment, ranges, ownership queries)
• unit tests for ClusterNode and ClusterState (creation, health checks, redirects)
• unit tests for all CLUSTER command parsing
• cargo clippy -- -D warnings passes
• cargo test -p ember-cluster -p ember-protocol all 203+ tests pass

design considerations

• used the exact CRC16 lookup table from redis source for guaranteed compatibility
• SlotMap uses a boxed array to avoid 128KB stack allocation
• NodeId wraps UUID v4 for cluster-wide uniqueness
• cluster bus port defaults to data port + 10000 (redis convention)
• CLUSTER KEYSLOT is fully functional; other cluster commands return stubs until full integration