feat(safety): drive command watchdog

### Summary

### Tasks
- Create `packages/monad-safety/watchdog.py`
- Enforce speed <= MAX_SAFE_SPEED_MS, no stale timestamps, deadman timeout.
- Integrate in orchestrator before executing drive.
- Emit telemetry on violations.

### Acceptance
Unit tests for violations; orchestrator returns 422; telemetry logged.


### Area

safety

### Priority

high