Spacing scale

Production writes spacing by hand, and almost every value is a multiple of 0.4rem — 1.6rem and 2.4rem are by far the most common. One base step of 0.4rem puts every spacing utility on that grid: p-4 is 1.6rem, gap-6 is 2.4rem. Page gutter is --grid-gap (1.6rem, production padding-main).