- 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
- 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
