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

All DoD criteria met:
- Rendezvous assignment deterministic (test_rendezvous_determinism)
- Minimal reshuffling on node add (test_minimal_reshuffling_on_add)
- 64/3/RF=1 distribution 18-26 shards per node (test_shard_distribution_64_3_rf1)
- Top-RF placement stable (test_top_rf_stability)
- write_targets returns RG × RF nodes (test_write_targets_count)
- query_group distributes evenly (test_query_group_distribution)
- covering_set returns one node per shard (test_covering_set_one_per_shard)
- merger passes all merge/facet/limit tests
- 96.7% code coverage (exceeds 90% requirement)

Coverage breakdown:
- router.rs: 96.76% (328/339 lines)
- topology.rs: 100% (142/142 lines)
- merger.rs: 95.45% (357/374 lines)

All 82 tests pass in ~100 seconds.

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

notes/miroir-cdo-phase-1-verification.md

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+# Phase 1 — Core Routing: Verification Complete
+
+## Date
+2026-05-09
+
+## Summary
+Phase 1 (Core Routing) implementation is complete with all DoD criteria met.
+
+## Files Verified
+- `crates/miroir-core/src/router.rs` — Rendezvous hash-based routing and shard assignment
+- `crates/miroir-core/src/topology.rs` — Topology management: node registry, groups, and health state
+- `crates/miroir-core/src/scatter.rs` — Scatter orchestration (stub implementation per Phase 1 scope)
+- `crates/miroir-core/src/merger.rs` — Result merger: combines shard results into a single response
+
+## DoD Criteria Verification
+
+### 1. Rendezvous Assignment Determinism ✅
+- **Test**: `test_rendezvous_determinism`
+- **Result**: Assignment is deterministic given fixed node list
+
+### 2. Minimal Reshuffling on Node Add ✅
+- **Test**: `test_minimal_reshuffling_on_add`
+- **Result**: Adding a 4th node in a 3-node group moves at most ~2 × (1/4) of shards
+
+### 3. Shard Distribution (64/3/RF=1) ✅
+- **Test**: `test_shard_distribution_64_3_rf1`
+- **Result**: Each node holds 18–26 shards
+
+### 4. Top-RF Placement Stability ✅
+- **Test**: `test_top_rf_stability`
+- **Result**: Top-RF placement changes minimally on add/remove
+
+### 5. Write Targets Count ✅
+- **Test**: `test_write_targets_count`
+- **Result**: `write_targets` returns exactly `RG × RF` nodes
+
+### 6. Query Group Distribution ✅
+- **Test**: `test_query_group_distribution`
+- **Result**: `query_group(seq, RG)` distributes evenly across groups
+
+### 7. Covering Set Correctness ✅
+- **Tests**: `test_covering_set_one_per_shard`, `test_covering_set_replica_rotation`
+- **Result**: `covering_set` returns exactly one node per shard with intra-group replica rotation
+
+### 8. Merger Functionality ✅
+- **Tests**: All `merger::tests` including:
+  - `test_global_sort_by_ranking_score`
+  - `test_offset_and_limit_applied_after_merge`
+  - `test_facet_counts_summed_across_shards`
+  - `test_estimated_total_hits_summed`
+  - `test_ranking_score_stripped_when_not_requested`
+  - `test_miroir_shard_always_stripped`
+- **Result**: All merge/facet/limit tests pass
+
+### 9. Code Coverage ✅
+- **router.rs**: 96.76% line coverage (328/339 lines)
+- **topology.rs**: 100% line coverage (142/142 lines)
+- **merger.rs**: 95.45% line coverage (357/374 lines)
+- **scatter.rs**: 0% (stub implementation per Phase 1 scope)
+- **Combined Phase 1 coverage**: 96.7% (exceeds 90% requirement)
+
+## Test Results
+All 82 tests pass in ~100 seconds:
+```
+running 82 tests
+test result: ok. 82 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
+```
+
+## Key Implementation Details
+
+### Rendezvous Hashing (router.rs)
+- Uses `twox_hash::XxHash64` with seed 42 for consistent scoring
+- `score(shard_id, node_id)` computes deterministic scores
+- `assign_shard_in_group` selects top-RF nodes by score
+- Group-scoped assignment prevents same-group replica placement
+
+### Topology (topology.rs)
+- `Topology` struct manages nodes grouped by `replica_group`
+- `NodeStatus` enum: Healthy, Joining, Draining, Failed
+- `Group` represents independent query pools
+
+### Merger (merger.rs)
+- Global sort by `_rankingScore`
+- Offset/limit applied after merge
+- Facet aggregation across shards
+- `_miroir_shard` always stripped
+- `_rankingScore` conditionally included
+- `estimatedTotalHits` summed across shards
+
+## Notes
+- Scatter implementation is intentionally stubbed per Phase 1 scope (wired in Phase 2)
+- All tests use deterministic fixtures for reproducibility
+- Coverage measured with `cargo-llvm-cov`