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.cpp

@@ -1,5 +1,7 @@
 #include "UIRenderCommands.h"
 
+#include <algorithm>
+
 namespace {
 std::vector<UIRenderCommand> commands;
 int canvas_width = 0;
@@ -34,3 +36,21 @@ void UIRenderRestoreClip() {
     clip_x1 = saved_x1; clip_y1 = saved_y1;
     clip_x2 = saved_x2; clip_y2 = saved_y2;
 }
+
+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) {
+    UIRenderCommand command{UIRenderCommand::Image, 0, 0, 0, 0, argb};
+    for (int i = 0; i < 4; ++i) {
+        command.qx[i] = x[i] + 0.5f;
+        command.qy[i] = y[i] + 0.5f;
+    }
+    command.x1 = std::min({command.qx[0], command.qx[1], command.qx[2], command.qx[3]});
+    command.y1 = std::min({command.qy[0], command.qy[1], command.qy[2], command.qy[3]});
+    command.x2 = std::max({command.qx[0], command.qx[1], command.qx[2], command.qx[3]});
+    command.y2 = std::max({command.qy[0], command.qy[1], command.qy[2], command.qy[3]});
+    command.text = UIRenderTextureName(texture);
+    command.quad = true;
+    command.su = su; command.sv = sv; command.eu = eu; command.ev = ev;
+    command.blend = blend;
+    UIRenderAdd(std::move(command));
+}