refactor(font): move test fixtures into a dedicated testing segment

Mock data lived in lib/ and was re-exported through the slice's
production index.ts, advertising fixtures as part of the public API.
Move it to a testing/ segment and drop it from index.ts; consumers now
import explicitly via the $entities/Font/testing subpath.

Repoints the four mock consumers (FontApplicator story, sizeResolver
and fontCatalogStore specs, comparisonStore test) to the new subpath.

src/entities/Font/index.ts

@@ -3,3 +3,6 @@ export * from './domain';
 export * from './lib';
 export * from './model';
 export * from './ui';
+
+// `./testing` is intentionally not re-exported: fixtures must not leak into the
+// production public API. Import them via `$entities/Font/testing`.