SimpleGrid

Composes with the classes `Box` computes instead of replacing them.

Columns of equal width. Unlike `Grid`, children declare no span: the count lives here. A number applies at every width; an object sets it per breakpoint from `base` upwards.

The element it paints. `ul` when the grid IS a list, so the count is announced.

How each cell sits in its column. Cells stretch by default, so a row of cards ends up the same height; anything else lets them keep their own.

Gap between columns, and between rows unless `verticalSpacing` overrides it.

Gap between rows when it should differ from `spacing`. Falls back to `spacing`.