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.

Animates the element in when it first scrolls into view. `true` takes the catalogue entrance; an object tunes it — `preset`, `distance`, `spring`, `duration`, `once`, `amount`, `rootMargin` and `index` for a stagger. It lives here rather than on each surface so that `Card`, `Paper`, `GradientBorder`, `Section` and anything else built on `Box` inherit one implementation. Without it `Box` stays a server component; with it the root swaps for a client shell and the children keep rendering on the server. The entrance is a CSS transition, not a JavaScript animation: it is the only motion in the catalogue that fires WHILE the user scrolls, and the compositor should own it.

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

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