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/index.ts

@@ -26,8 +26,8 @@
  */
 
 export {
+    getScrollBreadcrumbsStore,
     type NavigationAction,
-    scrollBreadcrumbsStore,
 } from './model';
 export {
     BreadcrumbHeader,

src/features/Breadcrumb/model/store/scrollBreadcrumbsStore.svelte.ts

@@ -167,6 +167,13 @@ class ScrollBreadcrumbsStore {
         this.#detachScrollListener();
     }
 
+    /**
+     * Tears down the observer and scroll listener. Call on store disposal.
+     */
+    destroy(): void {
+        this.#disconnect();
+    }
+
     /**
      * All tracked items sorted by index
      */
@@ -272,7 +279,17 @@ export function createScrollBreadcrumbsStore(): ScrollBreadcrumbsStore {
     return new ScrollBreadcrumbsStore();
 }
 
+let _scrollBreadcrumbsStore: ScrollBreadcrumbsStore | undefined;
+
 /**
- * Singleton scroll breadcrumbs store instance
+ * App-wide scroll breadcrumbs store, created on first access.
  */
-export const scrollBreadcrumbsStore = createScrollBreadcrumbsStore();
+export function getScrollBreadcrumbsStore(): ScrollBreadcrumbsStore {
+    return (_scrollBreadcrumbsStore ??= createScrollBreadcrumbsStore());
+}
+
+// test-only reset, so specs don't share observer/scroll state
+export function __resetScrollBreadcrumbsStore() {
+    _scrollBreadcrumbsStore?.destroy();
+    _scrollBreadcrumbsStore = undefined;
+}

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