Grid mode now drives a shared UiScrollable from its content + clips whole rows
to the panel (cells offset by -ScrollY, Visible toggled by whole-row clip,
mirroring UiText). Fill mode (toolbar single cell) unchanged. LayoutCells made
internal for tests; RowCount helper added.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
