Add comprehensive tests for the saveHistoryWithRetry function covering:
- Backup file creation from existing history
- Empty history serialization
- Single retry success
- Read-only directory failure with retries
- Concurrent saves with serialization via saveMu
- Snapshot isolation during save
Coverage: saveHistoryWithRetry 58.6% → 86.2%
Coverage: alerts package 87.4% → 87.8%