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/EterPythonLib/PythonGraphic.cpp

@@ -15,7 +15,7 @@
 void CPythonGraphic::SetViewport(float x, float y, float width, float height) { UIRenderSetClip(x, y, width, height); }
 void CPythonGraphic::RestoreViewport() { UIRenderRestoreClip(); }
 void CPythonGraphic::SetOmniLight() { MT_PLATFORM_STUB(); }
-long CPythonGraphic::GenerateColor(float, float, float, float) { MT_PLATFORM_STUB(); return 0; }
+long CPythonGraphic::GenerateColor(float r, float g, float b, float a) { return GetColor(r, g, b, a); }
 void CPythonGraphic::RenderUpButton(float, float, float, float) { MT_PLATFORM_STUB(); }
 void CPythonGraphic::RenderDownButton(float, float, float, float) { MT_PLATFORM_STUB(); }
 bool CPythonGraphic::SaveScreenShot(const char*) { MT_PLATFORM_STUB(); return false; }