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/GameEngine.h

@@ -10,6 +10,7 @@
 #include "HeadlessRenderer.h"
 #include "SceneTransition.h"
 #include "Profiler.h"
+#include "ImGuiConsole.h"
 #include <memory>
 #include <sstream>
 
@@ -62,6 +63,10 @@ private:
     int overlayUpdateCounter = 0;             // Only update overlay every N frames
     ProfilerOverlay* profilerOverlay = nullptr; // The actual overlay renderer
 
+    // ImGui console overlay
+    ImGuiConsole console;
+    bool imguiInitialized = false;
+
     void updateViewport();
 
     void testTimers();