- 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
- Create Engine structure to manage memtables, SSTables and WAL
- Implement MemTable to SSTable flush mechanism
- Add background flush goroutine for periodic flushing
- Build iterator system for reading from multiple data sources
- Create range-bounded iterators for queries
- Implement unified hierarchical iterator in iterator package
- Update TODO.md to mark Phase D as complete
