P4.2: Verify node addition with dual-write + paginated migration

Verified the P4.2 implementation is complete:
- All 6 integration tests pass (p42_node_addition.rs)
- All 14 cutover chaos tests pass
- All 8 topology chaos tests pass
- Core components: rebalancer.rs, migration.rs, rebalancer_worker/mod.rs
- Admin API: POST /_miroir/nodes endpoint

Acceptance criteria met:
- 3→4 node migration with 10K docs verified
- Chaos testing confirms dual-write catches late writes
- Performance bounds verified (≤total_docs/(Ng+1)×1.1)
- Log inspection confirms old node not queried after migration

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

notes/miroir-mkk.2.md

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+# P4.2 Node Addition: Dual-Write + Paginated Shard Migration - Verification
+
+## Summary
+
+Verified that the P4.2 node addition implementation is complete and fully functional. All acceptance criteria tests pass.
+
+## Implementation Verified
+
+### Core Components (Already Implemented)
+
+1. **`crates/miroir-core/src/rebalancer.rs`**:
+   - `add_node()` method creates node addition operation
+   - `compute_shard_moves_for_new_node()` determines affected shards
+   - `run_migration_task()` executes paginated migration with dual-write
+   - `HttpMigrationExecutor` implements actual HTTP-based document migration
+   - Delta pass support for catching stragglers
+
+2. **`crates/miroir-core/src/migration.rs`**:
+   - Complete migration state machine with all phases
+   - `MigrationCoordinator` for state transitions
+   - `begin_dual_write()` starts dual-write phase
+   - `begin_cutover()`, `complete_drain()`, `complete_cleanup()` for cutover sequence
+   - Safety validation (anti-entropy required if delta pass skipped)
+
+3. **`crates/miroir-core/src/rebalancer_worker/mod.rs`**:
+   - Background worker with leader lease
+   - `on_node_added()` handles topology events
+   - `drive_migrations()` advances migration state machine
+   - Job persistence via TaskStore
+
+4. **`crates/miroir-proxy/src/routes/admin_endpoints.rs`**:
+   - `add_node` API endpoint
+   - Integration with rebalancer worker via event channel
+
+### Integration Tests (All Pass)
+
+**`crates/miroir-core/tests/p42_node_addition.rs`**:
+1. `p42_node_addition_3_to_4_migration_10k_docs` - 10K docs migration test
+2. `p42_chaos_writes_during_migration_dual_write` - Dual-write chaos test
+3. `p42_performance_document_count_bounds` - Performance bounds verification
+4. `p42_log_inspection_old_node_not_queried_after_migration` - Log inspection test
+5. `p42_verify_dual_write_during_migration` - Dual-write verification
+6. `p42_pagination_limit_offset` - Pagination verification
+
+**`crates/miroir-core/tests/cutover_race.rs`**:
+- 14 chaos tests for cutover boundary conditions
+- All pass with anti-entropy and delta pass
+
+**`crates/miroir-core/tests/p4_topology_chaos.rs`**:
+- 8 topology chaos tests
+- All pass
+
+## Acceptance Criteria Status
+
+- [x] Integration test: 3-node → 4-node migration, 10K docs, each doc still retrievable by ID after migration
+- [x] Chaos: toggle writes on/off during migration; dual-write window catches all late writes
+- [x] Performance: migrating `~S/(Ng+1)` shards moves ≤ `total_docs / (Ng+1) × 1.1` docs (10% slack for dual-write dupes)
+- [x] The old node is not queried for the migrated shards after step 8 (verified via log inspection)
+
+## Test Results
+
+```
+running 6 tests (p42_node_addition)
+test p42_chaos_writes_during_migration_dual_write ... ok
+test p42_node_addition_3_to_4_migration_10k_docs ... ok
+test p42_log_inspection_old_node_not_queried_after_migration ... ok
+test p42_performance_document_count_bounds ... ok
+test p42_verify_dual_write_during_migration ... ok
+test p42_pagination_limit_offset ... ok
+test result: ok. 6 passed
+
+running 14 tests (cutover_race)
+test result: ok. 14 passed
+
+running 8 tests (p4_topology_chaos)
+test result: ok. 8 passed
+```
+
+Total: 28 tests passed, 0 failed.