Make DatasetStore the collision-free id authority

src/app/stores/DatasetStore.test.ts

@@ -186,3 +186,63 @@ describe('remove & view transitions', () => {
     expect(store().view).toBe('detail');
   });
 });
+
+describe('id assignment — the store is the collision-free id authority', () => {
+  test('add reassigns a fresh id even when two creates share a Date.now() id', () => {
+    // Simulate the tight-loop hazard: two datasets minted with the SAME injected id
+    // (what Date.now() yields within one millisecond). add must still give them
+    // distinct, monotonically increasing ids so they cannot collide in IndexedDB.
+    store().add(
+      createDataset({
+        name: 'A',
+        data: [{ a: 1 }],
+        format: 'json',
+        source: 'inline',
+        now: T,
+        id: 42,
+      }),
+    );
+    store().add(
+      createDataset({
+        name: 'B',
+        data: [{ a: 1 }],
+        format: 'json',
+        source: 'inline',
+        now: T,
+        id: 42,
+      }),
+    );
+    const ids = store().datasets.map((d) => d.id);
+    expect(new Set(ids).size).toBe(2);
+    // selectedId tracks the reassigned id of the most recent add, not the input 42.
+    expect(store().selectedId).toBe(store().datasets[0].id);
+  });
+
+  test('addDatasets gives a batch distinct ids past the existing maximum', () => {
+    store().add(
+      createDataset({ name: 'Seed', data: [{ a: 1 }], format: 'json', source: 'inline', now: T }),
+    );
+    const seedId = store().datasets[0].id;
+    store().addDatasets([
+      createDataset({
+        name: 'X',
+        data: [{ a: 1 }],
+        format: 'json',
+        source: 'inline',
+        now: T,
+        id: 42,
+      }),
+      createDataset({
+        name: 'Y',
+        data: [{ a: 1 }],
+        format: 'json',
+        source: 'inline',
+        now: T,
+        id: 42,
+      }),
+    ]);
+    const ids = store().datasets.map((d) => d.id);
+    expect(new Set(ids).size).toBe(3);
+    expect(Math.min(...ids.filter((i) => i !== seedId))).toBeGreaterThan(seedId);
+  });
+});