test(street-level): settle coverage before reading the ON/OFF pill

The pill step read #sl-status straight after the credit wait. Tiles
requested while Cesium painted the credit can flip it back to LOADING,
which failed CI on PR #1038 (actual 'BUTTON:LOADING:true'). Wait for
coverage to settle and the pill to read ON before asserting.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

scripts/qa-street-level.mjs

@@ -1321,6 +1321,13 @@ async function main() {
       await step(
         'the header ON/OFF pill switches the layer off and on',
         async () => {
+          // Tiles requested while the credit painted can flip the pill back
+          // to LOADING, so let coverage settle before reading it.
+          await waitForCoverage(page);
+          await settle(
+            page,
+            () => document.getElementById('sl-status').textContent === 'ON',
+          );
           assert.equal(
             await page.$eval('#sl-status', (node) =>
               [