refactor(breadcrumb): relocate Breadcrumb slice from entities to features

Breadcrumb is not a business aggregate — it is a scroll-tracking navigation
capability (NavigationWrapper registers page sections into a store), so it
belongs in the features layer, not entities. Move the whole slice and
repoint its three widget consumers. entities/ now holds only Font, a true
aggregate.

src/features/Breadcrumb/ui/BreadcrumbHeader/BreadcrumbHeader.svelte.test.ts

@@ -0,0 +1,11 @@
+import { render } from '@testing-library/svelte';
+import BreadcrumbHeader from './BreadcrumbHeader.svelte';
+
+const context = new Map([['responsive', { isMobile: false, isMobileOrTablet: false }]]);
+
+describe('BreadcrumbHeader', () => {
+    it('renders nothing when no sections have been scrolled past', () => {
+        const { container } = render(BreadcrumbHeader, { context });
+        expect(container.firstElementChild).toBeNull();
+    });
+});