Phase 1 (miroir-cdo): Core Routing verification complete

All DoD requirements verified:
- Rendezvous assignment deterministic ✅
- Minimal reshuffling on node add ✅
- Balanced shard distribution ✅
- write_targets returns RG × RF nodes ✅
- query_group distributes evenly ✅
- covering_set returns one node per shard ✅
- merger passes all merge/facet/limit tests ✅
- Coverage: router 95.4%, topology 100%, scatter 100%, merger 96.8% ✅

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

notes/miroir-cdo-phase1-verification.md

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+# Phase 1 (miroir-cdo): Core Routing Verification Summary
+
+## Definition of Done Status
+
+All Phase 1 Core Routing requirements have been verified complete:
+
+### Router Tests (router.rs) - All Pass ✅
+1. ✅ Rendezvous assignment is deterministic given fixed node list (`test_rendezvous_determinism`)
+2. ✅ Adding a 4th node in a 3-node group moves at most ~2 × (1/4) of shards (`test_minimal_reshuffling_on_add`)
+3. ✅ 64 shards / 3 nodes / RF=1 → each node holds 18–26 shards (`test_shard_distribution_64_3_rf1`)
+4. ✅ Top-RF placement changes minimally on add / remove (`test_top_rf_stability`)
+5. ✅ `write_targets` returns exactly `RG × RF` nodes (`test_write_targets_count`)
+6. ✅ `query_group(seq, RG)` distributes evenly (`test_query_group_distribution`)
+7. ✅ `covering_set` within a group returns exactly one node per shard (`test_covering_set_one_per_shard`)
+8. ✅ `covering_set` handles intra-group replica rotation (`test_covering_set_replica_rotation`)
+
+### Merger Tests (merger.rs) - All Pass ✅
+- Global sort by ranking score ✅
+- Offset and limit applied after merge ✅
+- Ranking score stripped when not requested ✅
+- Ranking score included when requested ✅
+- `_miroir_shard` always stripped ✅
+- Facet counts summed across shards ✅
+- Estimated total hits summed ✅
+- Processing time max across shards ✅
+- Degraded flag when shard fails ✅
+- Facet keys unique to one shard preserved ✅
+- Missing facet distribution handled gracefully ✅
+
+### Coverage (via lcov.info) ✅
+- **router.rs**: 95.4% (357/374 lines)
+- **topology.rs**: 100% (142/142 lines)
+- **scatter.rs**: 100% (122/122 lines)
+- **merger.rs**: 96.8% (328/339 lines)
+
+All core routing files meet the ≥ 90% coverage requirement.
+
+## Implementation Details
+
+### router.rs
+- `score(shard, node)`: Rendezvous scoring with XxHash64 (seed=42)
+- `assign_shard_in_group()`: RF assignment within a single replica group
+- `write_targets()`: Returns RG × RF nodes (one from each group)
+- `query_group()`: Round-robin group selection for queries
+- `covering_set()`: One node per shard with intra-group replica rotation
+- `shard_for_key()`: Primary key to shard mapping
+
+### topology.rs
+- `NodeId`: Unique node identifier
+- `NodeStatus`: Health state machine (Healthy/Degraded/Draining/Failed/Joining/Active/Removed)
+- `Node`: Node metadata with health tracking
+- `Group`: Replica group with node list
+- `Topology`: Cluster-wide topology with groups and nodes
+
+### scatter.rs
+- `Scatter` trait: Async fan-out orchestration
+- `StubScatter`: Stubbed implementation (wired in Phase 2)
+
+### merger.rs
+- `Merger` trait: Result merge interface
+- `MergerImpl`: Full implementation with global sort, facet aggregation, offset/limit
+- `StubMerger`: Empty result stub
+
+## Test Results
+- 105/106 tests pass in miroir-core
+- 1 unrelated test failure: `config::load::tests::test_from_yaml_valid_config` (config validation issue, not routing)
+- All 23 router/topology/scatter/merger tests pass