datannurpy v0.20.0a3

• add: metadata-first pattern. add_folder(create_folders=False) skips creating folders from disk and reuses id/folder_id from metadata/dataset.csv (matched by data_path). New on_unmatched parameter ("skip" / "warn" / "error") controls behavior for files with no metadata match.
• internal: new runtime field Dataset._match_path (absolute path used for scan↔metadata matching, not exported). data_path is now treated as the public link only.