- Introduced comprehensive tests:
- `test_buffer_open_nonexistent_save.cc`: Save after opening a non-existent file.
- `test_buffer_save.cc`: Save buffer contents to disk.
- `test_buffer_save_existing.cc`: Save after opening existing files.
- Implemented `PieceTable::WriteToStream()` to directly stream content without full materialization.
- Updated `Buffer::Save` and `Buffer::SaveAs` to use efficient streaming via `PieceTable`.
- Enhanced editing commands (`Insert`, `Delete`, `Replace`, etc.) to use PieceTable APIs, ensuring proper undo and save functionality.
