feat(tests): add chaos test scenarios and runbooks (plan §8, P9.4)
Add comprehensive chaos testing infrastructure for Miroir failure scenarios:
- **TestCluster** harness with chaos helpers:
- `kill_meili()` / `restart_meili()` for node failure simulation
- `apply_netem()` / `remove_netem()` for network delay injection
- `kill_miroir()` / `restart_miroir()` for orchestrator failure
- Docker-compose stack lifecycle management
- **6 chaos test scenarios** (all marked `#[ignore]`):
1. Kill 1 of 3 nodes (RF=2) → continuous search, no degraded header
2. Kill 2 of 3 nodes (RF=2) → 503 or partial results with degraded header
3. Kill 1 of 2 Miroir replicas → zero client-visible downtime
4. tc netem 500ms delay → searches slow but succeed, no errors
5. Restart killed node → Miroir detects recovery within health check interval
6. Kill node mid-rebalance → rebalancer pauses, resumes on recovery
- **Runbooks** in `tests/chaos/runbooks/scenario*.md`:
- Manual reproduction steps
- Expected observables (metrics, headers, errors)
- Recovery procedures
- HA vs single-instance differences
- Operator notes and common causes
- **Updated docker-compose files**:
- Added `CAP_NET_ADMIN` to all Meilisearch containers for tc netem support
Tests are slow (30+ seconds each) and require docker-compose. Run with:
cargo test --test chaos -- --ignored --test-threads=1
Closes: miroir-89x.4
304879d32a