P4.4: Add implementation summary note

## Retrospective
- **What worked:** The state machine approach with clear phase transitions (Initializing → Syncing → SyncComplete → Active) made the flow easy to understand and test. Separating the coordinator from the sync worker allowed for clean testing.
- **What didn't:** Initial implementation had the sync worker running in a tight loop; needed to add configurable intervals and proper timeout handling.
- **Surprise:** The query routing already filtered by group state, so the 'queries NOT routed to initializing groups' requirement was already satisfied by existing  logic.
- **Reusable pattern:** For future multi-phase operations, use a Coordinator + Worker pattern where the coordinator manages state/progress and the worker performs the actual work with periodic checkpoints.

notes/miroir-mkk.4.md

@@ -0,0 +1,48 @@
+# P4.4 Replica Group Addition: Initializing → Active
+
+## Summary
+
+This bead implements the "Adding a new replica group" flow from plan §2, enabling horizontal query scaling by adding new replica groups without interrupting existing queries.
+
+## Implementation
+
+### Core Components
+
+1. **`GroupAdditionCoordinator`** (`crates/miroir-core/src/group_addition.rs`)
+   - State machine for group addition: Initializing → Syncing → SyncComplete → Active
+   - Per-shard sync state tracking with round-robin source group selection
+   - Progress tracking and timeout handling
+
+2. **`GroupSyncWorker`** (`crates/miroir-core/src/group_sync_worker.rs`)
+   - Background worker that copies documents from existing groups to new group
+   - Paginated sync using `filter=_miroir_shard={id}`
+   - Handles source unavailability gracefully (pauses and resumes)
+
+3. **Admin API** (`crates/miroir-proxy/src/routes/admin_endpoints.rs`)
+   - `POST /_miroir/replica_groups` - Add new replica group
+   - `GET /_miroir/replica_groups/{id}/status` - Check sync progress
+   - `POST /_miroir/replica_groups/{id}/activate` - Mark group as active
+
+### Key Behaviors
+
+- **Query routing**: During sync, queries only route to `Active` groups (not `Initializing`)
+- **Write fan-out**: New writes immediately fan out to all groups including the new one
+- **Zero read interruption**: Existing groups continue serving queries throughout
+- **Round-robin source selection**: Spreads read load during sync across source groups
+
+## Acceptance Tests
+
+All 4 acceptance tests pass:
+1. ✓ During sync, query throughput on original group unchanged
+2. ✓ After `active`, queries distribute round-robin between groups
+3. ✓ Mid-sync writes present on both groups after sync
+4. ✓ Failed sync pauses and resumes when source returns
+
+## Files Modified
+
+- `crates/miroir-core/src/group_addition.rs` - New file
+- `crates/miroir-core/src/group_sync_worker.rs` - New file
+- `crates/miroir-core/tests/p44_replica_group_addition.rs` - New test file
+- `crates/miroir-proxy/src/routes/admin_endpoints.rs` - Added group addition endpoints
+- `crates/miroir-proxy/src/routes/admin.rs` - Added routes
+- `crates/miroir-proxy/src/main.rs` - Added sync worker background task