refactor(breadcrumb): replace scrollBreadcrumbsStore singleton with lazy accessor

Convert the eager scrollBreadcrumbsStore singleton to getScrollBreadcrumbsStore()
(+ __resetScrollBreadcrumbsStore for tests) and add a public destroy() that
disconnects the IntersectionObserver and scroll listener. Update the feature
barrel and consumers (BreadcrumbHeader, BreadcrumbHeaderSeeded, NavigationWrapper).

src/features/Breadcrumb/ui/NavigationWrapper/NavigationWrapper.svelte

@@ -6,9 +6,11 @@
 import { type Snippet } from 'svelte';
 import {
     type NavigationAction,
-    scrollBreadcrumbsStore,
+    getScrollBreadcrumbsStore,
 } from '../../model';
 
+const scrollBreadcrumbsStore = getScrollBreadcrumbsStore();
+
 interface Props {
     /**
      * Navigation index