* tar: add explicit zstd compression support
Add an explicit --zstd flag for create, list, and extract so
.tar.zst archives work end to end.
Keep the scope narrow by requiring the flag instead of trying to
autodetect compression. Add CLI coverage and end-to-end tests for
creating, listing, and extracting zstd-compressed archives.
* tar: update benchmarks for compression mode
Pass CompressionMode::None from the existing tar benchmarks after the archive operation helpers gained explicit compression arguments.
Keep the operation helpers and CompressionMode public so the external benchmark target can continue importing them through the uu_tar library.
Kevin Burke
a997edac52