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/ImGuiConsole.h

@@ -0,0 +1,56 @@
+#pragma once
+
+#include <string>
+#include <vector>
+#include <deque>
+
+/**
+ * @brief ImGui-based debug console for Python REPL
+ *
+ * Provides an overlay console that can execute Python code
+ * without blocking the main game loop. Activated by grave/tilde key.
+ */
+class ImGuiConsole {
+public:
+    ImGuiConsole();
+
+    // Core functionality
+    void render();              // Render the console UI
+    void toggle();              // Toggle visibility
+    bool isVisible() const { return visible; }
+    void setVisible(bool v) { visible = v; }
+
+    // Configuration (for Python API)
+    static bool isEnabled() { return enabled; }
+    static void setEnabled(bool e) { enabled = e; }
+
+    // Input handling
+    bool wantsKeyboardInput() const;  // Returns true if ImGui wants keyboard
+
+private:
+    void executeCommand(const std::string& command);
+    void addOutput(const std::string& text, bool isError = false);
+
+    // State
+    bool visible = false;
+    static bool enabled;  // Global enable/disable (for shipping games)
+
+    // Input buffer
+    char inputBuffer[1024] = {0};
+
+    // Output history
+    struct OutputLine {
+        std::string text;
+        bool isError;
+        bool isInput;  // True if this was user input (for styling)
+    };
+    std::deque<OutputLine> outputHistory;
+    static constexpr size_t MAX_HISTORY = 500;
+
+    // Command history for up/down navigation
+    std::vector<std::string> commandHistory;
+    int historyIndex = -1;
+
+    // Scroll state
+    bool scrollToBottom = true;
+};