fix(electron): show the main window once its document has loaded; enforce J1 IPC and mutation counters (#1828)

* fix(electron): show the main window once its document has loaded; enforce J1 IPC and mutation counters

Re-lands #1788, which merged into #1782's branch after #1782 had already
reached master, so none of it is on master.

The hidden main window was shown on ready-to-show only. On Linux under
X11, when the startup scripts run before the window's first frame, the
next frame comes about a second later: nothing is on screen and the
splash's requestAnimationFrame waits, so J1's first card came ~940 ms
after load instead of ~480 ms in most runner launches (18 bridge calls /
1,031-1,033 DOM mutations instead of 15 / 558).

The window is now shown at ready-to-show or the main frame's
did-finish-load, whichever comes first, with the splash colour as its
background so showing before the first paint does not flash. The journey
gate keeps the app's did-finish-load listeners away from its about:blank
detour, as it already does for ready-to-show.

Three dispatched runs on this branch (37192092882, 37192097790,
37192103151) read 15 calls and 558 mutations in all 18 iterations,
stable: true. Both become baselines (slack 0), and the Performance
journeys job checks them with check-journey-ratchet.mjs --only.

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

* chore(perf): record the evidence PR of the J1 runtime baselines

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

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: 4gray <fourgray@proton.me>

tools/performance/journey-baselines.json

@@ -9,6 +9,24 @@
                 "updatedAt": "2026-10-01",
                 "evidencePr": 1775,
                 "measuredWith": "pnpm nx build web && pnpm run perf:initial-bytes"
+            },
+            "renderer.ipcCallsToFirstCard": {
+                "value": 15,
+                "unit": "calls",
+                "slack": 0,
+                "updatedAt": "2026-10-04",
+                "evidencePr": 1828,
+                "evidenceRun": "https://github.com/4gray/iptvnator/actions/runs/37192092882",
+                "measuredWith": "pnpm run perf:journeys (ubuntu-latest, xvfb), identical in 3 runs"
+            },
+            "renderer.domMutationsToFirstCard": {
+                "value": 558,
+                "unit": "mutations",
+                "slack": 0,
+                "updatedAt": "2026-10-04",
+                "evidencePr": 1828,
+                "evidenceRun": "https://github.com/4gray/iptvnator/actions/runs/37192092882",
+                "measuredWith": "pnpm run perf:journeys (ubuntu-latest, xvfb), identical in 3 runs"
             }
         }
     }