A step opens a tab the way a person has to, through the menu when it is hidden

CI run 35 was red: the suite died after step 15 with `widget with text
"Container" not found`. Nothing in the editor broke — the tenth tab (#73) pushed
Container behind the `»` button at the test window's 1280 px, exactly the case
#84 was built for, and three steps still clicked the tab name directly.

openTab(t, tab) in tools/e2e/lib/scaffold.mjs asks tabRow() whether the tab is
hidden and goes through the menu when it is, so the fix lives in one place rather
than in every step that opens a tab. The LABELS record moves there as TAB_LABELS,
since both the helper and 14-overflow need it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tools/e2e/16-container.mjs

@@ -1,4 +1,5 @@
 import { instanceChild } from '../lib/builders.mjs';
+import { openTab } from './lib/scaffold.mjs';
 
 /**
  * The Container tab: editing a template with the scene view's tools. Relies on the container step
@@ -38,8 +39,8 @@ export async function containerTab(t) {
   await sleep(200);
 
   // 16a. The tab opens on the container the project has, and its header says which one.
-  await clickText('Container');
-  await sleep(500);
+  await openTab(t, 'container');
+  await sleep(300);
   const template = await json(`__gm.state.project.assets.find(a => a.id === '${containerId}').objects`);
   const count = template.length;
   check(