test(library): read the shelf roll at a walking pace so the landing step can see it

A roll read at 150ms a frame finishes before the flush clock can be told apart from a batch-only flush. At 900ms the step that watches the frames land has 14 counts to show for the reading instead of 3, and a build that only hands the rows over when a batch is full fails it.

docker/frontend/scripts/tree-scan-check.mjs

@@ -675,7 +675,7 @@ step(
 await page.evaluate(async () => {
   sessionStorage.removeItem('check.lists');
   localStorage.removeItem('check.hold');
-  localStorage.setItem('check.slow', '150');
+  localStorage.setItem('check.slow', '900');
   localStorage.setItem('check.roll', 'BigRoll');
   const db = await new Promise((resolve, reject) => {
     const req = indexedDB.open('recipescam-library');