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

All DoD criteria verified:
- Rendezvous assignment deterministic (1000 runs test)
- Minimal reshuffling on add/remove (≤ 2×1/N bound)
- Uniform distribution (64/3/RF1: 15-27 shards/node)
- Top-RF placement stable
- write_targets returns RG×RF nodes
- query_group distributes evenly
- covering_set returns one node per shard
- merger passes all tests
- miroir-core coverage: 91.80% (≥90% required)

Test results: 151 passed, 0 failed

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

notes/miroir-cdo-verification.md

@@ -0,0 +1,94 @@
+# Phase 1 — Core Routing: Final Verification
+
+## Date
+2026-05-09
+
+## Definition of Done Verification
+
+### ✅ All Criteria Met
+
+1. **Rendezvous assignment determinism** ✅
+   - `acceptance_determinism_1000_runs`: Verifies identical assignments across 1000 runs
+   - `test_rendezvous_determinism`: Basic determinism check
+
+2. **Minimal reshuffling on node add** ✅
+   - `acceptance_reshuffle_bound_on_add`: 64 shards, 3→4 nodes moves ≤ 32 shards (2 × 1/4 × 64)
+   - `test_minimal_reshuffling_on_add`: General case verification
+
+3. **Uniform shard distribution** ✅
+   - `acceptance_uniformity_64_shards_3_nodes_rf1`: 64 shards, 3 nodes, RF=1 → each node holds 15–27 shards
+   - `test_shard_distribution_64_3_rf1`: Same test with debug output
+
+4. **Top-RF placement stability** ✅
+   - `acceptance_rf2_placement_stability`: RF=2 placement changes minimally on add/remove
+   - `test_top_rf_stability`: Top-RF changes verification
+   - `acceptance_reshuffle_bound_on_remove`: Remove node behavior
+
+5. **write_targets returns RG × RF nodes** ✅
+   - `test_write_targets_count`: Verifies exact count, uniqueness, and per-group contribution
+
+6. **query_group distributes evenly** ✅
+   - `test_query_group_distribution`: Round-robin distribution verification
+
+7. **covering_set returns one node per shard** ✅
+   - `test_covering_set_one_per_shard`: Exact shard count verification
+   - `test_covering_set_replica_rotation`: Intra-group replica rotation
+
+8. **merger functionality** ✅
+   - All merger tests pass: global sort, offset/limit, facet aggregation, field stripping
+   - `test_global_sort_by_ranking_score`
+   - `test_offset_and_limit_applied_after_merge`
+   - `test_facet_counts_summed_across_shards`
+   - `test_miroir_shard_always_stripped`
+   - `test_ranking_score_stripped_when_not_requested`
+   - `test_offset_limit_pagination_reconstruction`
+   - `test_stable_serialization_same_input_same_json`
+   - And 15+ more merger tests
+
+9. **miroir-core ≥ 90% line coverage** ✅
+   - router.rs: 96.20% line coverage
+   - topology.rs: 100.00% line coverage
+   - scatter.rs: 100.00% line coverage
+   - merger.rs: 94.67% line coverage
+   - **Total miroir-core: 91.80% line coverage**
+
+## Test Results
+- **Total tests**: 151 passed, 0 failed
+- All router tests passing (18+ tests)
+- All topology tests passing (35+ tests)
+- All scatter tests passing (5+ tests)
+- All merger tests passing (20+ tests)
+
+## Implementation Summary
+
+### router.rs
+- `score(shard_id, node_id)`: XxHash64 with seed 0 (matches Meilisearch Enterprise)
+- `assign_shard_in_group()`: Deterministic HRW assignment
+- `write_targets()`: Returns RG × RF nodes for writes
+- `query_group()`: Round-robin group selection
+- `covering_set()`: One node per shard with replica rotation
+- `shard_for_key()`: Primary key to shard mapping
+
+### topology.rs
+- `NodeId`: Unique node identifier
+- `NodeStatus`: 7-state machine with valid transitions
+- `Node`: Health state management, write eligibility
+- `Group`: Replica group with node list
+- `Topology`: Cluster-level groups, nodes, RF, shards
+
+### scatter.rs
+- `Scatter` trait: Fan-out orchestration (stubbed for Phase 2 wiring)
+- `StubScatter`: Default stub implementation
+- Data structures: `ScatterRequest`, `ScatterResponse`, `NodeResponse`
+
+### merger.rs
+- `merge()`: Global sort, offset/limit, facet aggregation
+- Field stripping: `_miroir_*` always, `_rankingScore` conditional
+- Binary heap optimization for large fan-out
+- `Merger` trait with `MergerImpl` and `StubMerger`
+
+## Conclusion
+Phase 1 Core Routing is **complete** and meets all Definition of Done criteria.
+The implementation provides deterministic, coordination-free routing with
+minimal reshuffling on topology changes — the foundation for all subsequent
+phases.