- Add create_backup() to backup files before patching
- Add apply_patch_to_file() to apply unified diff
- Add restore_from_backup() for rollback
- Add cleanup_backup() to remove backup files
- Add 15 unit tests for all patch operations
- Create src/diff_viewer.py with parse_diff function
- Parse unified diff into DiffFile and DiffHunk dataclasses
- Extract file paths, hunk headers, and line changes
- Add unit tests for diff parser