Mark

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

Which scale the highlight is drawn from — the 200 step fills the background and the 900 step writes the text, which is what keeps the pair readable in any of them. Name the scale only (`"success"`), never a step: `"success.500"` pins BOTH ends to that one step and the text disappears into its own background.

The element it paints. A real `mark` is what tells assistive tech this run is called out, so changing it turns the highlight into decoration the reader never hears about.