test(perf): widen the J3 mutation slack to 20

The rerun of a8505e0f9's journeys read 733 with one fifth-call iteration at
750, over the 749 limit; the run before it read 736 throughout. The usual
value moves between 731 and 736 within one commit and a fifth bridge call
adds exactly 17, so the band tops out at 753: slack 20.

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

tools/performance/journey-baselines.json

@@ -123,12 +123,12 @@
             "renderer.domMutationsToPlaying": {
                 "value": 733,
                 "unit": "mutations",
-                "slack": 16,
+                "slack": 20,
                 "autoTighten": false,
                 "updatedAt": "2026-10-13",
                 "evidencePr": 1817,
                 "evidenceRun": "https://github.com/4gray/iptvnator/actions/runs/38311314484",
-                "measuredWith": "pnpm run perf:journeys (ubuntu-latest, xvfb); 731-736 across master commits since #1817, 748 with a fifth bridge call"
+                "measuredWith": "pnpm run perf:journeys (ubuntu-latest, xvfb); 731-736 across runs since #1817, +17 (up to 753) when a fifth bridge call lands before playing"
             },
             "renderer.httpRequestsToPlaying": {
                 "value": 2,