fix(swingset): scan mosaic source for stylex css extraction

The postcss plugin's package auto-discovery only recognizes a dependency as a
StyleX package when its manifest lists `@stylexjs/stylex` under dependencies,
peerDependencies, or optionalDependencies. Moving it to `@clerk/ui`'s
devDependencies silently dropped `packages/ui/src/mosaic` from extraction, so
no `:root { --cl-* }` token defaults were emitted and the preview deploy
rendered unstyled. Name the source in `include` instead of relying on discovery.

Author: alexcarpenter

packages/swingset/postcss.config.mjs

@@ -21,8 +21,12 @@ const isDev = process.env.NODE_ENV !== 'production';
 // `runtimeInjection` (see `next.config.mjs`) injects the *fresh* atom at runtime under a new
 // content hash, which HMR tracks. The stale extracted atom is dead CSS; the `:root` token
 // defaults never change mid-session, so they stay correct.
+//
+// `include` names the Mosaic source explicitly: the plugin's auto-discovery ignores
+// `devDependencies`, which is where `@clerk/ui` keeps `@stylexjs/stylex`.
 const stylexExtraction = {
   '@stylexjs/postcss-plugin': {
+    include: ['src/**/*.{js,jsx,mjs,cjs,ts,tsx,mts,cts}', `${uiRoot}/src/mosaic/**/*.{js,jsx,mjs,cjs,ts,tsx,mts,cts}`],
     useCSSLayers: true,
     babelConfig: {
       babelrc: false,