- Add `ProgressReporter` trait with `start`, `update`, and `finish` methods for customizable progress tracking during operations.
- Implement `ProgressInfo` struct for detailed progress state reporting, including operation, progress, and context information.
- Create a no-op implementation `NoopProgressReporter` for cases where progress reporting is not needed.
- Enhance `pkg6` with structured error handling using `Pkg6Error` and new diagnostic variants for I/O, JSON, FMRI, and logging errors.
- Update `Image` functionality to leverage progress reporting during catalog downloads and include robust publisher management.
- Introduce `TestProgressReporter` for unit tests to validate progress reporting integration.
- Modify `rebuild` and `refresh` methods to take `&self` instead of `&mut self`.
- Replace mutable repository instances in `pkg6dev`, `pkg6repo`, and core library with immutable ones.
- Introduce `debug` logging for enhanced traceability during metadata rebuild and refresh.
- Extract and simplify catalog rebuild logic into a dedicated `rebuild_catalog` method.
- General cleanup: remove redundant methods, reorganize logic, and refine comments across affected files.
- 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- Clean up redundant and dead code, enhance comments, and replace outdated logic for file handling and metadata operations.
