docs: note that CI's two engines share one app

Which is why a shared-selector fault can be green on chromium and red
on webkit in the same run, and how to reproduce it locally.

.planning/NOTES.md

@@ -3751,3 +3751,13 @@ class names into every ancestor query.
 It also only appears in a suite that has *done* something — the
 sections are empty on a fresh app, so this cannot be reproduced by
 opening Settings and looking.
+
+**And it appears on the second engine, not the first.** CI runs
+chromium then webkit against **one app**, so a spec that scans in the
+chromium pass leaves a finished job the webkit pass then trips over.
+Three specs used that selector; two failed locally and the third
+(`failure-voice.spec.ts`) was green on chromium and red on webkit in
+the same run. Reproducing it locally is running the suite twice against
+one `make dev-headless` — which is worth doing for any change that
+leaves state behind, since it is the only place a cross-engine order
+dependency shows up.

e2e/specs/failure-voice.spec.ts

@@ -36,9 +36,16 @@ test.describe('a failed binding says so', () => {
     // Libraries is the one section that starts expanded (H-22), so ask
     // the disclosure what state it is in rather than assuming one — a
     // blind click used to expand it and now collapses it.
+    //
+    // By role and name, not by `.header`: since #27 the section also
+    // contains a `job-panel`, and an open `job-details-drawer` inside
+    // it carries the same class. That only bites once a job exists,
+    // which is why it showed up on the *second* engine of a CI run and
+    // not the first.
     const disclosure = page
       .locator('config-section[heading="Libraries"]')
-      .locator('.header');
+      .getByRole('button', { name: 'Libraries' })
+      .first();
 
     if ((await disclosure.getAttribute('aria-expanded')) === 'false') {
       await disclosure.click();