- Remove redundant MergedIterator (was just an alias for HierarchicalIterator)
- Add IsTombstone method to all iterators to detect deletion markers
- Enhance tombstone tracking in compaction manager with preservation options
- Fix SSTable reader to properly handle tombstone entries
- Update engine tests to directly verify tombstone behavior
- Update TODO.md to mark merged iterator task as complete
- Implement config package with serializable configuration and manifest
- Implement WAL with durability guarantees and fragmentation support
- Fix mutex deadlock bug in WAL sync operations
- Add comprehensive tests for both packages
