feat(bulk-edit): add bulkSelection store backed by SvelteSet

Module-singleton live accumulator: selection persists across pagination
and route changes within /documents and /enrich. Cleared on successful
bulk save or via Alles aufheben.

Refs #225

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/stores/bulkSelection.svelte.spec.ts

@@ -0,0 +1,53 @@
+import { afterEach, describe, expect, it } from 'vitest';
+import { bulkSelectionStore } from './bulkSelection.svelte';
+
+describe('bulkSelectionStore', () => {
+	afterEach(() => bulkSelectionStore.clear());
+
+	it('starts empty', () => {
+		expect(bulkSelectionStore.size).toBe(0);
+	});
+
+	it('toggle adds an id when absent', () => {
+		bulkSelectionStore.toggle('a');
+		expect(bulkSelectionStore.has('a')).toBe(true);
+		expect(bulkSelectionStore.size).toBe(1);
+	});
+
+	it('toggle removes an id when present', () => {
+		bulkSelectionStore.add('a');
+		bulkSelectionStore.toggle('a');
+		expect(bulkSelectionStore.has('a')).toBe(false);
+	});
+
+	it('add and remove update size', () => {
+		bulkSelectionStore.add('a');
+		bulkSelectionStore.add('b');
+		expect(bulkSelectionStore.size).toBe(2);
+		bulkSelectionStore.remove('a');
+		expect(bulkSelectionStore.size).toBe(1);
+		expect(bulkSelectionStore.has('b')).toBe(true);
+	});
+
+	it('add is idempotent', () => {
+		bulkSelectionStore.add('a');
+		bulkSelectionStore.add('a');
+		expect(bulkSelectionStore.size).toBe(1);
+	});
+
+	it('setAll replaces the selection', () => {
+		bulkSelectionStore.add('a');
+		bulkSelectionStore.add('b');
+		bulkSelectionStore.setAll(['c', 'd', 'e']);
+		expect(bulkSelectionStore.size).toBe(3);
+		expect(bulkSelectionStore.has('a')).toBe(false);
+		expect(bulkSelectionStore.has('c')).toBe(true);
+	});
+
+	it('clear empties the selection', () => {
+		bulkSelectionStore.add('a');
+		bulkSelectionStore.add('b');
+		bulkSelectionStore.clear();
+		expect(bulkSelectionStore.size).toBe(0);
+	});
+});