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/BreadcrumbHeaderSeeded.svelte

@@ -0,0 +1,49 @@
+<script>
+import { onMount } from 'svelte';
+import { scrollBreadcrumbsStore } from '../../model';
+import BreadcrumbHeader from './BreadcrumbHeader.svelte';
+
+const sections = [
+    { index: 100, title: 'Introduction' },
+    { index: 101, title: 'Typography' },
+    { index: 102, title: 'Spacing' },
+];
+
+/** @type {HTMLDivElement} */
+let container;
+
+onMount(() => {
+    for (const section of sections) {
+        const el = /** @type {HTMLElement} */ (container.querySelector(`[data-story-index="${section.index}"]`));
+        scrollBreadcrumbsStore.add({ index: section.index, title: section.title, element: el }, 96);
+    }
+
+    /*
+     * Scroll past the sections so IntersectionObserver marks them as
+     * scrolled-past, making scrolledPastItems non-empty and the header visible.
+     */
+    setTimeout(() => {
+        window.scrollTo({ top: 2000, behavior: 'instant' });
+    }, 100);
+
+    return () => {
+        for (const { index } of sections) {
+            scrollBreadcrumbsStore.remove(index);
+        }
+        window.scrollTo({ top: 0, behavior: 'instant' });
+    };
+});
+</script>
+
+<div bind:this={container} style="height: 2400px; padding-top: 900px;">
+    {#each sections as section}
+        <div
+            data-story-index={section.index}
+            style="height: 400px; padding: 2rem; background: #f5f5f5; margin-bottom: 1rem;"
+        >
+            {section.title} — scroll up to see the breadcrumb header
+        </div>
+    {/each}
+</div>
+
+<BreadcrumbHeader />