test(performance): add J4 search journey

Measures typing a six-character query into the header search box on
/workspace/search until the global search results settle, on a profile
with the M3U fixture and the mock's existing 12,000-item `large` Xtream
catalog. Counters: bridge calls and SQL statements per search (with a
per-keystroke breakdown), serial IPC depth, DOM mutations, change-detection
ticks, layout shift and long tasks; wall-clock last keystroke to settled
and first keystroke to first result.

Runs in the existing journeys target and the warn-only CI job, whose
summary now prints the per-keystroke table. Moves J3's picsum artwork
blocker into a shared 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, playback) is six fresh Electron processes plus
-        # one seeding run.
+        # (launch, open-source, playback, search) 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.