test(perf): enforce J3 renderer.domMutationsToPlaying (733, slack 16)

#1817 windowed the EPG timeline: on the runner the counter fell from 6,182
to 731, long tasks from 1 to 0, and click->playing P50 from 221-341 ms to
196-215 ms, which validates it (Principle 3). It reads 731-736 across master
commits and 748 when a fifth bridge call lands before playing, so the entry
carries slack 16; the regression it guards is ~5,000 mutations above that.

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

.github/workflows/ci.yml

@@ -351,6 +351,7 @@ jobs:
                   --only open-source/main.mockHttpRequestsToSettled
                   --only open-source/renderer.ipcCallsToFirstPage
                   --only open-source/renderer.layoutShiftScore
+                  --only playback/renderer.domMutationsToPlaying
                   --only playback/renderer.httpRequestsToPlaying
                   --only playback/renderer.layoutShiftScore
                   --only search/renderer.ipcCallsPerSearch