Add benchmarks for core operations, migration edge case tests, improved
buffer I/O tests, and developer guide
- Introduced `test_benchmarks.cc` for performance benchmarking of key
operations in `PieceTable` and `Buffer`, including syntax highlighting
and iteration patterns.
- Added `test_migration_coverage.cc` to provide comprehensive tests for
migration of `Buffer::Rows()` to `PieceTable` APIs, with edge cases,
boundary handling, and consistency checks.
- Enhanced `test_buffer_io.cc` with additional cases for save/load
workflows, file handling, and better integration with the core API.
- Documented architectural details and core concepts in a new
`DEVELOPER_GUIDE.md`. Highlighted design principles, code
organization, and contribution workflows.
- Added detailed journaling system (`SwapManager`) for crash recovery, including edit recording and replay.
- Integrated recovery prompts for handling swap files during file open flows.
- Implemented swap file cleanup, checkpointing, and compaction mechanisms.
- Added extensive unit tests for swap-related behaviors such as recovery prompts, file pruning, and corruption handling.
- Updated CMake to include new test files.
- Added `QtFrontend`, `QtRenderer`, and `QtInputHandler` for Qt-based UI rendering and input handling.
- Implemented support for theming, font customization, and palette overrides in GUITheme.
- Renamed and refactored ImGui-specific components (e.g., `GUIRenderer` -> `ImGuiRenderer`).
- Added cross-frontend integration for commands and visual font picker.
