tests: flip the dropzone smoke test to assert the new gated behavior

The old test expected `.clip-track-dropzone` to always be present.
The dropzone is now hidden until the clip library has at least one
clip, so the blank smoke scene must not contain it.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/devtools/tests.ts

@@ -1509,9 +1509,13 @@ export function registerAllTests(): void {
       expect(brand).toBeTruthy()
     })
 
-    test('timeline has new-track dropzone', () => {
+    test('new-track dropzone is hidden while the clip library is empty', () => {
+      // Regression: the dropzone used to always render and show "Drop a
+      // clip here" even when the library had no clips. Timeline now gates
+      // `.clip-track-dropzone` behind `scene.clips.length > 0`. The live
+      // smoke scene starts blank, so the dropzone must be absent.
       const dz = document.querySelector('.clip-track-dropzone')
-      expect(dz).toBeTruthy()
+      expect(dz).toBeNull()
     })
 
     test('timeline has add-group button', () => {