Two field-schema additions consumed by Amplify's Hero overlay style:

- `required: true` field opt renders a required-marker asterisk on the
  label (text/textarea/number/select/radio/asset), static or via
  resolve_fields/2.
- `:radio` `default:` opt seeds the checked option when the stored value
  is blank, so pre-existing configs without a stored value reflect the
  default instead of rendering with nothing selected.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>