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/shared/lib/transitions/springySlideFade/springySlideFade.ts

@@ -3,10 +3,6 @@ import type {
     TransitionConfig,
 } from 'svelte/transition';
 
-function elasticOut(t: number) {
-    return Math.pow(2, -10 * t) * Math.sin((t - 0.075) * (2 * Math.PI) / 0.3) + 1;
-}
-
 function gentleSpring(t: number) {
     return 1 - Math.pow(1 - t, 3) * Math.cos(t * Math.PI * 2);
 }