Churn simulator: validate 80% completion at 30% churn over 72h

## Description

Per spec T144c and whitepaper Phase 2 requirement: "Over a 72-hour continuous run, 80% of submitted test jobs complete correctly with 30% simulated node churn."

## Requirements

- Build configurable churn simulator: random node kill/rejoin at configurable rate
- Integrate with multi-node test harness
- Track job completion rates under churn
- Validate checkpoint/resume works across node failures
- Run for configurable duration (target: 72 hours for Phase 2)
- Report completion rate, data loss events, and recovery metrics

## Success Criteria

- [ ] Simulator can kill/rejoin nodes at configurable rate (target: 30% churn)
- [ ] Job completion rate ≥ 80% at 30% churn over 72 hours
- [ ] Zero data loss events during churn
- [ ] Checkpoint/resume works correctly across node failures
- [ ] Metrics reported: completion rate, recovery time, data loss count

## Testing (Principle V)

- Run on 20+ node testbed with real hardware
- 30% churn rate → verify 80% job completion
- Verify zero data loss during churn
- Measure checkpoint/resume latency under various churn rates