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/EterLib/UIRenderCommands.h

@@ -12,8 +12,32 @@ struct UIRenderCommand {
     std::uint32_t argb;
     float clip_x1 = 0, clip_y1 = 0, clip_x2 = 0, clip_y2 = 0;
     std::string text;
+    // Image only: text is the texture's file, drawn as the quad (vertices 0..3 = TL, TR, BL, BR)
+    // with texture coordinates su,sv..eu,ev; x1..y2 is the quad's bounding box.
+    bool quad = false;
+    float qx[4] = {}, qy[4] = {};
+    float su = 0, sv = 0, eu = 1, ev = 1;
+    int blend = 0; // CGraphicExpandedImageInstance::RENDERING_MODE_*
 };
 
+// D3DXCOLOR (0..1 floats) -> the 0xAARRGGBB the commands carry.
+template <class Color>
+std::uint32_t UIRenderColor(const Color& color) {
+    auto channel = [](float value) -> std::uint32_t {
+        return static_cast<std::uint32_t>((value < 0 ? 0 : value > 1 ? 1 : value) * 255.0f + 0.5f);
+    };
+    return (channel(color.a) << 24) | (channel(color.r) << 16) | (channel(color.g) << 8) | channel(color.b);
+}
+
+class CGraphicTexture;
+std::string UIRenderTextureName(const CGraphicTexture* texture);
+
+// A textured quad from 40250's TPDTVertex[4] (TL, TR, BL, BR) positions and texture coordinates.
+// PORT: D3D8 puts pixel centres on integers, which is why 40250 subtracts 0.5 from every vertex; the
+// Godot canvas puts them on .5, so the quad is shifted back by +0.5 here.
+void UIRenderAddImage(const CGraphicTexture* texture, const float x[4], const float y[4],
+                      float su, float sv, float eu, float ev, std::uint32_t argb, int blend = 0);
+
 void UIRenderSetSize(int width, int height);
 void UIRenderGetSize(unsigned* width, unsigned* height);
 void UIRenderBeginFrame();