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/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;
+}