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/lib/sizeResolver/createFontRowSizeResolver.test.ts

@@ -14,6 +14,7 @@ vi.mock('@chenglou/pretext', async () => {
         layout: vi.fn(actual.layout),
     };
 });
+import { mockUnifiedFont } from '$entities/Font/testing';
 import {
     beforeEach,
     describe,
@@ -22,7 +23,6 @@ import {
     vi,
 } from 'vitest';
 import type { FontLoadStatus } from '../../model/types';
-import { mockUnifiedFont } from '../mocks';
 import { createFontRowSizeResolver } from './createFontRowSizeResolver';
 
 // Fixed-width canvas mock: every character is 10px wide regardless of font.