Smooth the page's text in the foundation, not per surface

The showcase and the error pages set grayscale font smoothing on
themselves, but the foundation did not, so an application that dropped
Tailwind's antialiased class rendered heavier text on macOS than the
package's own pages, silently. base.css now sets it on the page; the
showcase's and error page's copies go, and the guard tells a leftover
antialiased class to go too. Decided with the user (plan step 46).

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

tests/Feature/ShowcaseAssetsTest.php

@@ -21,7 +21,7 @@ it('serves the bundle as long-cached CSS at the hash of its own content', functi
         ->assertHeader('Cache-Control', 'immutable, max-age=31536000, public');
 
     expect($response->getContent())
-        ->toContain('[data-md-showcase]')
+        ->toContain('[data-md-showcase-brand]')
         ->toContain('[data-md-pane]')
         ->toContain('@layer material.reset, material.tokens');
 });