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/python_host_node.h

@@ -30,6 +30,8 @@ public:
 	static godot::String run_main_script(const godot::String &command_line);
 	// One line of Python in the same __main__ dictionary. "" when it ran.
 	static godot::String run_line(const godot::String &source);
+	// True while system.py is inside app.Loop(); ui_update() then runs one CPythonApplication::Process().
+	static bool is_app_looping();
 	static void set_ui_size(int width, int height);
 	static void ui_mouse_move(int x, int y);
 	static void ui_mouse_button(int button, bool pressed, int x, int y);