Add stemedb-cluster crate implementing horizontal scaling:
- SWIM-based membership protocol for node discovery and failure detection
- Consistent hashing (jump hash) for subject-to-shard routing
- Range management with dynamic split (>64MB) and merge (<20MB) operations
- Stateless HTTP gateway for client request routing via axum
- Meta-range gossip merge for cluster-wide metadata propagation
Includes restrictive CORS policy, proper error propagation from routing,
replica cache invalidation on node failure, and 84 tests (57 unit + 27
integration). Raft MV coordination deferred per design decision.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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