vertical (default) stacks rows down the page. horizontal lays them out
along the inline axis behind a scroll container, with the rail as a
horizontal axis and the content stacked away from it.
Orientation composes with placement rather than replacing it: under
horizontal, [placement] selects which side of the axis a row's body
sits on, so alternate puts cards above and below in turn.
Named orientation rather than direction because [direction] is
already the sort order on this component.