feat: M8 phases 7-10 — gRPC transport, cluster server, scatter-gather, multi-node UAT
Delivers the distributed fabric's network layer and HTTP cluster surface:
**m8p7: tidal-net crate (gRPC transport)**
- GrpcTransport implementing Transport trait via tonic 0.12
- Per-peer circuit breaker (Closed/Open/HalfOpen), mutual TLS via rustls
- Boxed error types (clippy-clean), graceful mutex recovery, debug_assert
against calling block_on from tokio context
- Proto: WalShipping service (ShipSegment, StreamSegments stub, Heartbeat)
- 19 tests: contract, mTLS, reconnection, multi-node UAT, benchmarks
**m8p8: cluster subcommand + HTTP routes**
- ClusterState wrapping SimulatedCluster with region name mapping
- Routes: /health, /cluster/status, /cluster/promote, /partition, /heal
- Data routes: /items, /embeddings, /signals, /feed, /search (region-aware)
- Ranking profiles wired through ClusterConfig to all cluster nodes
- Topology YAML config, docker/cluster/Dockerfile (ENTRYPOINT+CMD, non-root)
**m8p9: scatter-gather query routing**
- Entity-sharded writes via Knuth multiplicative hash
- Scatter-gather RETRIEVE and SEARCH with deadline propagation (50ms-5ms)
- Partial failure: degraded=true with unavailable_shards metadata
- 6 tests: distribution, determinism, multi-shard retrieve, degraded
partial results, deadline propagation, scatter-gather search
**m8p10: gRPC transport integration tests**
- 8 tests over real gRPC: replication convergence, idempotent replay,
mixed signals, 3-node fan-out, partition/heal, degraded follower, perf
- Documented as tier-2 (in-process+gRPC); tier-3 multi-process pending
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