test(perf): add the J3 playback journey

J3 clicks a live channel of an Xtream portal and ends at the built-in
HTML5 player's first `playing` event, with `loadedmetadata` as a
secondary phase. It follows J2: every iteration is a fresh J1 launch on
a copy of a profile seeded through the app's dialogs, and the click
happens after the app has settled in the portal's first live category.

The portal is the mock's `live-fallback` account, whose `.ts` live URLs
serve the local H.264/AAC MPEG-TS fixture that mpegts.js plays through
MSE on every platform. The marketing accounts' local live bytes are
zero-filled and never reach `playing`. Seeding selects the HTML5 player
and the `ts` stream format; the catalog's picsum.photos logos are
cancelled from the test side so no request leaves the machine.

Counters: renderer.ipcCallsToPlaying, renderer.httpRequestsToPlaying,
renderer.domMutationsToPlaying, renderer.layoutShiftScore and
renderer.longTasks; wall-clock click->loadedmetadata and click->playing.
renderer.ipcSerialDepthToPlaying is listed as unavailable until the
serial-depth helper lands. No baseline yet.

The probe gains a media-event terminal; J2's pre-click settle moves to
journey-click-settle.ts so both journeys share it unchanged, and the
probe spec's jsdom fixtures move to a shared test helper.

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

.github/workflows/ci.yml

@@ -296,8 +296,8 @@ jobs:
         if: needs.performance-journeys-scope.outputs.run == 'true'
         runs-on: ubuntu-latest
         # The electron-performance build is the bulk of the time; each journey
-        # (launch, open-source) is six fresh Electron processes plus one
-        # seeding run.
+        # (launch, open-source, playback) is six fresh Electron processes plus
+        # one seeding run.
         timeout-minutes: 30
         # Warn-only for the first two weeks of plan item B3: a failure is
         # visible on the run but does not fail the workflow.