2V0-f: run the real system.py app loop in Godot

- system.py runs app.Loop() on the script fiber; each UIUpdate is one
  CPythonApplication::Process(). Logo -> Login, popup ESC/click and
  app.Exit teardown (launcher cleared first, as in 40250 Main) work.
- Image commands are base-texture quads with UVs (a .sub shares its parent
  texture); ImageInstance/ExpandedImageInstance OnRender and SetRenderingRect
  ported verbatim, so scale/rotation/rendering rect are drawn.
- CGraphicBase::GetColor / grp.GenerateColor ported (ThinBoard background).
- port.app_loop test, python_host_test app-loop checks, MT_TEST_MODE=python_ui.
- TODO: .fnt/CGraphicText font pipeline, ExpandedImage blend modes.

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

extension/src/platform/ScriptLib/PythonBoot.h

@@ -29,7 +29,18 @@ bool IsRunning();
 // __COMMAND_LINE__ and RunFile("system.py"). 2V0-e registers temporary observable gameplay modules;
 // they are replaced by the real modules in later 2V slices.
 // False leaves *error set to what the launcher reported through LogBox (platform/EterBase/LogBox.h).
+//
+// PORT: system.py runs on a script thread. RunMainScript returns once prototype.RunApp() has reached
+// app.Loop() (IsAppLooping) or the script has ended; from then on every AppFrame() runs exactly one
+// CPythonApplication::Process() while the host thread waits, so the two never run concurrently.
 bool RunMainScript(const char* lpCmdLine, std::string* error);
+bool IsAppLooping();
+bool AppFrame();
+
+// Used by the platform CPythonApplication: Loop() calls AppLoopWait() before each Process() and stops
+// when it returns false; Exit()/Abort() call AppRequestQuit() (40250: PostQuitMessage).
+bool AppLoopWait();
+void AppRequestQuit();
 
 // CPythonLauncher::RunLine, with the reported text handed back rather than only shown. The host's own
 // check uses it to prove the interpreter and the standard library work in-process on a device that has
@@ -42,6 +53,7 @@ void SetUISize(int width, int height);
 void UIMouseMove(int x, int y);
 void UIMouseButton(int button, bool pressed, int x, int y);
 void UIKey(int key, bool pressed);
+// While IsAppLooping(), UIUpdate is AppFrame() and UIRender keeps the commands that Process() drew.
 void UIUpdate();
 void UIRender();