FIX: Align ZipPath.root with pathlib

[Mirochill]

Summary

• Make ZipPath.root describe the root portion of the internal archive path, matching pathlib behavior.
• Add ZipPath.zipfile as the explicit accessor for the underlying ZipFile handle.
• Update internal uses and add a regression covering root and zipfile on child paths.

Validation

• Not run locally.
• git diff --check
• git diff --cached --check
• git diff --check HEAD~1..HEAD
• git show --stat --oneline HEAD -- archive_path/zip_path.py tests/test_basic.py

Closes #18