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/BreadcrumbHeader/BreadcrumbHeader.svelte

@@ -14,9 +14,10 @@ import { cubicOut } from 'svelte/easing';
 import { slide } from 'svelte/transition';
 import {
     type BreadcrumbItem,
-    scrollBreadcrumbsStore,
+    getScrollBreadcrumbsStore,
 } from '../../model';
 
+const scrollBreadcrumbsStore = getScrollBreadcrumbsStore();
 const breadcrumbs = $derived(scrollBreadcrumbsStore.scrolledPastItems);
 const responsive = getContext<ResponsiveManager>('responsive');
 

src/features/Breadcrumb/ui/BreadcrumbHeader/BreadcrumbHeaderSeeded.svelte

@@ -1,8 +1,10 @@
 <script>
 import { onMount } from 'svelte';
-import { scrollBreadcrumbsStore } from '../../model';
+import { getScrollBreadcrumbsStore } from '../../model';
 import BreadcrumbHeader from './BreadcrumbHeader.svelte';
 
+const scrollBreadcrumbsStore = getScrollBreadcrumbsStore();
+
 const sections = [
     { index: 100, title: 'Introduction' },
     { index: 101, title: 'Typography' },

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