Matching x-heights exactly overcorrects: real Courier New sits at ~0.42 em
against DM Sans' 0.51, so an exact match scaled it 21% up and its capitals
and ascenders towered over everything else. The correction is now capped at
6%, and each preview normalizes its own stack rather than a shared default
so it shows the size the app will actually render.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>