Play the guest's changed demo in e2e 70 (#428)

The test plan of #428 asks a guest to change the demo and play it,
with no request that writes reaching the server. e2e 70 only changed
it.

- 70f now plays the changed demo and stops it before it counts the
  requests.

Tests: e2e 70 (atx) ALL PASSED, 13 checks, mode "play", no writes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tools/e2e/70-guest.mjs

@@ -203,16 +203,19 @@ export async function guest(t) {
       JSON.stringify(picker),
     );
 
-    // 70f. A change stays in the tab: well past the autosave's second, nothing has gone out.
+    // 70f. A change, played, stays in the tab: well past the autosave's second, nothing has gone out.
     await clickText('+');
     await pickAssetType(t, 'Sprite');
+    await t.play();
+    const mode = await json('__gm.state.mode');
+    await t.stop();
     await sleep(2500);
     const changed = await json('__gm.state.project.assets.length');
     const copy = await (await send('GET', `demos/${token}`, { as: 'nobody' })).json();
     check(
-      "a guest's change stays in the tab: no request that writes, and the copy on the server as it was",
-      changed === 3 && writes.length === 0 && copy.project.assets.length === 2,
-      JSON.stringify({ changed, writes, served: copy.project.assets.length }),
+      'a guest changes the demo and plays it: no request that writes, and the copy on the server as it was',
+      mode === 'play' && changed === 3 && writes.length === 0 && copy.project.assets.length === 2,
+      JSON.stringify({ mode, changed, writes, served: copy.project.assets.length }),
     );
 
     // 70g. Sign in… goes back to the sign-in screen; the demo picked again is the copy as it was.