P1.1 Rendezvous hash primitives - verification complete

Verify that the three core rendezvous hash primitives (score, assign_shard_in_group, shard_for_key) are correctly implemented in miroir_core::router.

All implementations match the specification:
- score: Uses XxHash64::with_seed(0) with canonical (shard_id, node_id) order
- assign_shard_in_group: Group-scoped assignment with score sort and lexicographic tie-breaking
- shard_for_key: Uses XxHash64::with_seed(0) to hash primary_key

All 26 acceptance tests pass:
- Determinism across 1000 runs
- Reshuffle bounds on add/remove
- Uniformity distribution (15-27 shards per node)
- RF=2 placement stability
- shard_for_key fixture verification

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

notes/miroir-cdo.1.md

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+# P1.1 Rendezvous Hash Primitives - Verification Complete
+
+## Task Status
+**COMPLETE** - All three required functions were already implemented in `/home/coding/miroir/crates/miroir-core/src/router.rs`
+
+## Implementations Verified
+
+### 1. `score(shard_id: u32, node_id: &str) -> u64`
+- Uses `XxHash64::with_seed(0)` (line 15)
+- Canonical concatenation order: `(shard_id, node_id)` (lines 16-17)
+- Returns `u64` hash value (line 18)
+
+### 2. `assign_shard_in_group(shard_id: u32, group_nodes: &[NodeId], rf: usize) -> Vec<NodeId>`
+- Computes score for each node in the group (line 30)
+- Sorts by score descending (line 33)
+- Breaks ties lexicographically on node_id (line 34)
+- Returns top `rf` nodes (lines 37-40)
+
+### 3. `shard_for_key(primary_key: &str, shard_count: u32) -> u32`
+- Uses `XxHash64::with_seed(0)` (line 73)
+- Hashes primary_key (line 74)
+- Returns `hash % shard_count` as u32 (line 75)
+
+## Acceptance Tests (All Passed ✓)
+
+1. **Determinism** - 1000 randomized runs all produce identical assignments
+2. **Reshuffle bound on add** - 64 shards, 3→4 nodes: ≤ 32 edges differ
+3. **Reshuffle bound on remove** - 64 shards, 4→3 nodes: ~32 edges differ
+4. **Uniformity** - 64 shards, 3 nodes, RF=1: each node holds 15-27 shards
+5. **RF=2 placement stability** - Top-2 nodes change minimally on add/remove
+6. **shard_for_key fixture** - Verified against known test vectors
+7. **Tie-breaking** - Lexicographic on node_id for identical scores
+8. **Canonical order** - (shard_id, node_id) produces different hash than (node_id, shard_id)
+
+## Test Results
+```
+running 26 tests
+test result: ok. 26 passed; 0 failed; 0 ignored
+```
+
+## Key Design Constraints Verified
+- Hash function: `twox-hash` (XxHash family) ✓
+- Seed: 0 (matches Meilisearch Enterprise) ✓
+- Concatenation order: (shard_id, node_id) ✓
+- Group-scoped assignment: prevents same-group replicas ✓
+- Tie-breaking: lexicographic on node_id ✓
+
+## Files Modified
+None - implementations were already correct and complete.