- add regression coverage for BaseException cleanup in atomic_json_write - add dedicated atomic_yaml_write tests, including interrupt cleanup - document why BaseException is intentional in both helpers
Extract the duplicated temp-file + fsync + os.replace pattern from batch_runner.py (1 instance) and process_registry.py (2 instances) into a shared utils.atomic_json_write() function. Add 12 tests for atomic_json_write covering: valid JSON, parent dir creation, overwrite, crash safety (original preserved on error), no temp file leaks, string paths, unicode, custom indent, concurrent writes. Add 12 tests for batch_runner checkpoint behavior covering: _save_checkpoint (valid JSON, last_updated, overwrite, lock/no-lock, parent dirs, no temp leaks), _load_checkpoint (missing file, existing data, corrupt JSON), and resume logic (preserves prior progress, different run_name starts fresh).
