Heading

Headings where level and size are separate, and body text that can be muted.

`level` picks both the tag and the size, but `as` separates them again. That
matters more than it looks: heading level is *structure* — the outline a
screen reader navigates by — while size is appearance. A page sometimes needs
an `h2` that looks small, and the heading order should not break for it.

`Text` is the paragraph: `muted` for the lead, the help text, the column
label. Three sizes, and `as` when it needs to be a `span`.