Add radius tokens and replace bare rounded classes

The theme inherited Tailwind's radius scale silently, so `--radius-md` meant one
thing in the editor's stylesheet and whatever the framework last shipped
everywhere else. Declaring the three rungs we use gives the scale one owner: a
change lands in one place, and `check-tokens` can see a misspelt rung now that
`--radius-` has left its `EXTERNAL` bypass list.

The values are Tailwind's own, so nothing moves. `sm` and `md` are what the
editor stylesheet already read; `lg` is the one utility site. `xs` and `xl` are
not declared because nothing uses them.

`--radius-*: initial` first: `@theme` merges with Tailwind's rather than
replacing it, so a rung we do not declare goes on resolving to a value we never
chose. Reaching for one should draw nothing.

The bare `rounded` utility is spelled `rounded-sm` at ten call sites. Both
resolve to `0.25rem`, so this is a rename rather than a change — `rounded` is
the v3 name and does not appear in the scale this file now owns.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>