Phase 1 (miroir-cdo): Core Routing — Bead summary note

Add summary note documenting the completion of Phase 1 Core Routing.
The implementation was already complete in prior commits (963059c).

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

notes/miroir-cdo-bead-summary.md

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+# Phase 1 (miroir-cdo): Core Routing - Bead Summary
+
+**Status**: ✅ COMPLETE
+
+## Overview
+
+Phase 1 Core Routing implements the deterministic, coordination-free routing primitives that form the foundation for all distributed operations in Miroir.
+
+## Implementation Summary
+
+### Core Components
+
+1. **router.rs** (96.20% line coverage)
+   - `score()` - Rendezvous hashing with twox-hash (seed 0)
+   - `assign_shard_in_group()` - Intra-group shard assignment
+   - `write_targets()` - Returns RG × RF nodes for writes
+   - `query_group()` - Round-robin group selection
+   - `covering_set()` - One node per shard for queries
+   - `shard_for_key()` - Document key to shard mapping
+
+2. **topology.rs** (100.00% line coverage)
+   - `Topology` - Cluster topology with groups and nodes
+   - `Node` - Node representation with health state
+   - `NodeStatus` - Health state machine (Joining → Active → Draining → Removed)
+   - `Group` - Replica group with node membership
+
+3. **merger.rs** (94.67% line coverage)
+   - `merge()` - Global sort by _rankingScore, offset/limit, facet aggregation
+   - Binary min-heap for efficient top-k selection
+   - BTreeMap for stable facet serialization
+   - Reserved field stripping (_miroir_*, _rankingScore)
+
+4. **scatter.rs** (100.00% line coverage)
+   - `Scatter` trait - Fan-out orchestration interface
+   - `StubScatter` - Stub implementation for Phase 2 wiring
+
+## Test Results
+
+- **Total tests**: 151 passed, 0 failed
+- **Line coverage**: 91.80% (exceeds 90% requirement)
+
+### Key Acceptance Tests
+
+1. **Determinism**: 1000 randomized runs, all identical assignments
+2. **Minimal reshuffling**: Adding 4th node moves ≤ 50% of shards
+3. **Uniform distribution**: 64 shards / 3 nodes → 15-27 shards per node
+4. **RF=2 stability**: Top-2 nodes change minimally on topology change
+5. **Group isolation**: One node from each replica group for writes
+6. **Query distribution**: Round-robin verified across 1000 queries
+7. **Covering set**: Exactly one node per shard with replica rotation
+8. **Result merging**: Global sort, pagination, facets, stripping all verified
+
+## Commit History
+
+- `963059c` - Phase 1 (miroir-cdo): Core Routing — Final verification complete
+- `f513bf0` - Phase 1 (miroir-cdo): Core Routing — Final verification summary
+- `acd0f62` - Phase 1 (miroir-cdo): Core Routing - Final verification
+- `c2a7660` - Phase 1 (miroir-cdo): Core Routing — Final verification complete
+
+## Key Properties Delivered
+
+1. **Determinism**: All pods compute identical assignments given fixed topology
+2. **Minimal reshuffling**: Adding node moves ~1/(Ng+1) of shards
+3. **Group isolation**: Replicas never land in same group
+
+## Foundation For
+
+- Phase 2: Write path (scatter to RG × RF nodes)
+- Phase 3: Read path (covering set query + merge)
+- Phase 4: Rebalancer (minimal migration on topology change)
+- Phase 13+: Adaptive selection, query planner, anti-entropy