Mock `El` now exposes `dataset` as a Proxy that syncs property
writes back to `attributes["data-*"]`, and `setAttribute("data-*", ...)`
populates the backing dataset with camelCase key. That way
`#target.dataset.val = "new"` updates the `data-val` attribute,
letting the swap command read/write the property correctly.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>