refactor: remove unused vars and dead code

Cleanup surfaced once the oxlint config actually loads (no-unused-vars).

- drop dead locals/imports/params (cachedOffsetTop, elasticOut, key,
  unused type imports, unused test imports; _-prefix unused mock params)
- createVirtualizer: keep the _version read (reactive subscription inside
  $derived.by) but bind it to _v so it is not flagged
- scrollBreadcrumbsStore.test: keep the removeEventListener mock side
  effect, drop the unread spy binding

src/entities/Font/model/store/fontLifecycleManager/utils/loadFont/loadFont.test.ts

@@ -71,7 +71,7 @@ describe('loadFont', () => {
     it('throws FontParseError when font.load() rejects', async () => {
         const loadError = new Error('parse failed');
         const MockFontFace = vi.fn(
-            function(this: any, name: string, buffer: BufferSource, options: FontFaceDescriptors) {
+            function(this: any, _name: string, _buffer: BufferSource, _options: FontFaceDescriptors) {
                 this.load = vi.fn().mockRejectedValue(loadError);
             },
         );

src/entities/Font/testing/stores.mock.ts

@@ -21,11 +21,7 @@
  */
 
 import type { UnifiedFont } from '$entities/Font/model/types';
-import type {
-    QueryKey,
-    QueryObserverResult,
-    QueryStatus,
-} from '@tanstack/svelte-query';
+import type { QueryStatus } from '@tanstack/svelte-query';
 import {
     UNIFIED_FONTS,
     generateMockFonts,