test(tui): align viewport snapshot key test with quantization

- keep 8-row key binning for scroll jitter stability and update the assertion to match runtime behavior

ui-tui/src/__tests__/viewportStore.test.ts

@@ -33,6 +33,6 @@ describe('viewportStore', () => {
       top: 13,
       viewportHeight: 5
     })
-    expect(viewportSnapshotKey(snap)).toBe('0:13:5:40:3')
+    expect(viewportSnapshotKey(snap)).toBe('0:16:5:40:3')
   })
 })