feat(core): Add xs size variant to Radio

[jaydgoss]

TLDR

Adds an xs size variant to the core Radio component with a 12px outer diameter and a 6px inner dot, alongside the existing sm (20px) and md (24px) sizes.

This new variant is going to be used in the context of an updated version of the project creation flow, the design of which calls for a 12px text label next to a radio button with an outer width of 12px. If I were to use existing sm radio button, it would appear out of proportion with the text label.

Details

The new xs variant is wired through the size prop union and radioConfig. Its 6px inner dot keeps the 1:2 inner-to-outer ratio used by the other sizes. The variant is covered in the snapshot tests across both themes and checked/unchecked states, and documented in the component stories.

The auto-generated Code Connect mapping (radio.figma.tsx) is left unchanged, since its size enum maps to Figma component property values that cannot be added from code.

[github-actions]

Story previews

Preview the stories changed in this PR on the Vercel deployment:

• app/components/core/radio/radio.mdx

_{Preview deployment: https://sentry-e826fy53o.sentry.dev}