- Deleted `GapBuffer` class and its API implementations.
- Removed `AppendBuffer` selector and conditional `KTE_USE_PIECE_TABLE` macros.
- Eliminated legacy support in buffer APIs, file I/O, benchmarks, and correctness tests.
- Updated guidelines and comments to reflect PieceTable as the default and only buffer backend.
- Normalize path handling for buffer operations, supporting tilde expansion and absolute paths.
- Introduce `DisplayNameFor` to uniquely resolve buffer display names, minimizing filename clashes.
- Add new commands: `ShowWorkingDirectory` and `ChangeWorkingDirectory`.
- Refine keybindings and enhance existing commands for improved command flow.
- Adjust GUI and terminal renderers to display total line counts alongside filenames.
- Update coding style to align with project guidelines.
