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>

UPGRADE.md

@@ -138,7 +138,8 @@ component the views render, opening with the layer statement every package style
 either: layout components (`<x-scaffold>`, `<x-pane>`, `<x-stack>`, `<x-row>`, `<x-grid>`, the
 canonical layouts) take M3's spacing tokens and breakpoints as props, a small set of `md-type-*`
 and `md-ink-*` classes covers text on plain elements, and `--md-sys-*` custom properties serve the
-rest of an application's own stylesheet.
+rest of an application's own stylesheet. The foundation smooths text in grayscale, as Tailwind's
+`antialiased` class did: drop the class.
 
 ### 7. New in 2.0.0