feat: Add ImGui-based developer console overlay

Integrates Dear ImGui for an in-game debug console that replaces the
blocking Python REPL. Press ~ (grave/tilde) to toggle the console.

Features:
- Python code execution without blocking the game loop
- Output capture with color coding (yellow=input, red=errors, gray=output)
- Expression results show repr() automatically
- Command history navigation with up/down arrows
- Word wrapping for long output lines
- Auto-scroll that doesn't fight manual scrolling
- mcrfpy.setDevConsole(bool) API to disable for shipping

Technical changes:
- Update imgui submodule to v1.89.9 (stable)
- Update imgui-sfml submodule to 2.6.x branch (SFML 2.x compatible)
- Add ImGui sources to CMakeLists.txt with OpenGL dependency
- Integrate ImGui lifecycle into GameEngine
- Add ImGuiConsole class for console overlay

closes #36, closes #65, closes #75

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/McRFPy_API.h

@@ -81,7 +81,10 @@ public:
     
     // Profiling/metrics
     static PyObject* _getMetrics(PyObject*, PyObject*);
-    
+
+    // Developer console
+    static PyObject* _setDevConsole(PyObject*, PyObject*);
+
     // Scene lifecycle management for Python Scene objects
     static void triggerSceneChange(const std::string& from_scene, const std::string& to_scene);
     static void updatePythonScenes(float dt);