- 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.
- Implement unit tests to cover `Image` creation, metadata management, saving/loading, and error handling.
- Add separate constructors for `Full` and `Partial` image types (`new_full`, `new_partial`).
- Introduce robust path handling methods (`metadata_dir`, `image_json_path`) for consistent metadata structure.
- Enhance error diagnostics with `InvalidPath` variant in `ImageError`.
- Update `Cargo.toml` for test module inclusion.
