Windows: every start ended itself after 3 s (1.3.0, 1.3.1)

Win32Window::Create calls Destroy() before it makes the window, which
ran FlutterWindow::OnDestroy and armed the 3 s exit timer at launch: the
window showed, then the process ended with code 0, no crash, no close
lines. The timer is only armed with an engine to tear down. Found and
fixed by Fun Maker's agent on Windows (a debugger caught the exit on the
timer's thread).

The smoke runs missed it (they closed at ~4 s anyway): the app now stays
up 10 s, and the scripts require 'closing' and 'quitting' from its own
close.

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

docs/ARCHITECTURE.md

@@ -2293,7 +2293,9 @@ app kills itself (`Process.killPid`, no exit handlers; it only gets there
 if the close did not go through); the runners end the process 15 s after
 the first close request (Linux: a `delete-event` handler connected before
 window_manager's; Windows: the first `WM_CLOSE`), and Windows also 3 s
-after the window is destroyed, while the engine and plugins go.
+after the window is destroyed, while the engine and plugins go (only
+with an engine: `Win32Window::Create` runs `OnDestroy` before the window
+exists, and a timer armed there ended every start at 3 s).
 
 ## Logging