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/ui/FontApplicator/FontApplicator.stories.svelte

@@ -23,7 +23,7 @@ const { Story } = defineMeta({
 </script>
 
 <script lang="ts">
-import { mockUnifiedFont } from '$entities/Font/lib/mocks';
+import { mockUnifiedFont } from '$entities/Font/testing';
 import type { ComponentProps } from 'svelte';
 
 const fontUnknown = mockUnifiedFont({ id: 'nonexistent-font-xk92z', name: 'Nonexistent Font Xk92z' });