- Update `FileBackend` to store files in a multi-level directory structure (`xx/yy/hash`) based on hash prefixes.
- Adjust method signatures and logic across `FileBackend`, `pkg6dev`, and `pkg6repo` to support mutable repository operations.
- Add test cases and scripts (`test_file_structure.rs`, `test_repo_operations.sh`) to validate the new file structure.
- Refactor catalog manager to use `RefCell` for interior mutability to enable lazy initialization.
- Clean up redundant and dead code, enhance comments, and replace outdated logic for file handling and metadata operations.
