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/helpers/createVirtualizer/createVirtualizer.svelte.ts

@@ -156,7 +156,7 @@ export function createVirtualizer<T>(
     const offsets = $derived.by(() => {
         const count = options.count;
         // Implicit dependency on version signal
-        const v = _version;
+        const _v = _version;
         const result = new Float64Array(count);
         let accumulated = 0;
         for (let i = 0; i < count; i++) {
@@ -180,7 +180,7 @@ export function createVirtualizer<T>(
         // this derivation when the items array is replaced!
         const { count, data } = options;
         // Implicit dependency
-        const v = _version;
+        const _v = _version;
         if (count === 0 || containerHeight === 0 || !data) {
             return [];
         }
@@ -268,7 +268,6 @@ export function createVirtualizer<T>(
                 return rect.top + scrollY;
             };
 
-            let cachedOffsetTop = 0;
             let rafId: number | null = null;
             containerHeight = typeof window !== 'undefined' ? window.innerHeight : 0;
 
@@ -292,14 +291,12 @@ export function createVirtualizer<T>(
             const handleResize = () => {
                 containerHeight = window.innerHeight;
                 elementOffsetTop = getElementOffset();
-                cachedOffsetTop = elementOffsetTop;
                 handleScroll();
             };
 
             // Initial setup
             requestAnimationFrame(() => {
                 elementOffsetTop = getElementOffset();
-                cachedOffsetTop = elementOffsetTop;
                 handleScroll();
             });