ci(perf): enforce J1 ipcCallsToFirstCard and domMutationsToFirstCard

With the main window shown at did-finish-load, J1 takes one path on the
runner: three dispatched runs (36928706097, 36928716010, 36928725392)
read 15 bridge calls and 559 DOM 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.

The journeys document records the race, its cause, the before/after
wall-clock and which other J1 counters are now stable.

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

docs/architecture/workspace-shell.md

@@ -432,15 +432,24 @@ Startup window mode (`Settings.startupWindowMode`, issue #1455):
 3. `fullscreen` is the `BrowserWindow` constructor option: on Windows/Linux
    the window is created hidden and enters fullscreen before its first
    paint. macOS ignores the option while the window is hidden (an NSWindow
-   only toggles fullscreen once it is on screen), so `ready-to-show` repeats
+   only toggles fullscreen once it is on screen), so the first show repeats
    the request with `setFullScreen(true)` right after `show()` wherever
    `isFullScreen()` is still false — never unconditionally, or the
    platforms that honoured the option would animate a second toggle. The
    saved bounds stay spread into the options — they are the normal bounds
    the window returns to, and the close handler keeps persisting
-   `getNormalBounds()`. `maximized` calls `maximize()` inside
-   `ready-to-show` right before `show()`, never earlier: `maximize()` on a
-   hidden window shows it, and a blank window would flash.
+   `getNormalBounds()`. `maximized` calls `maximize()` right before the
+   first `show()`, never earlier: `maximize()` on a hidden window shows it,
+   and a blank window would flash. That first show happens at
+   `ready-to-show` or the main frame's `did-finish-load`, whichever comes
+   first (`services/main-window-first-show.ts`): on Linux a hidden window
+   whose startup scripts ran before its first frame gets the next one about
+   a second later, so `ready-to-show` alone left the window off screen and
+   the splash's animation frame waiting. At `did-finish-load` the inline
+   splash is parsed, and the window's `backgroundColor` is the splash colour
+   (`MAIN_WINDOW_BACKGROUND_COLOR`, keep it in sync with `#initial-splash`
+   in `apps/web/src/index.html`), so showing before the first paint does
+   not flash.
 4. `iptvnator --fullscreen` (read via `app.commandLine.hasSwitch`, so it can
    sit anywhere in argv; the playlist-path extractor already skips every
    `-`-prefixed argument) forces `fullscreen` for that launch only and is