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.cpp

@@ -6,6 +6,8 @@
 #include "Resources.h"
 #include "Animation.h"
 #include "Timer.h"
+#include "imgui.h"
+#include "imgui-SFML.h"
 #include <cmath>
 
 GameEngine::GameEngine() : GameEngine(McRogueFaceConfig{})
@@ -31,8 +33,13 @@ GameEngine::GameEngine(const McRogueFaceConfig& cfg)
         window->create(sf::VideoMode(1024, 768), window_title, sf::Style::Titlebar | sf::Style::Close | sf::Style::Resize);
         window->setFramerateLimit(60);
         render_target = window.get();
+
+        // Initialize ImGui for the window
+        if (ImGui::SFML::Init(*window)) {
+            imguiInitialized = true;
+        }
     }
-    
+
     visible = render_target->getDefaultView();
     
     // Initialize the game view
@@ -116,6 +123,12 @@ void GameEngine::cleanup()
         McRFPy_API::game = nullptr;
     }
     
+    // Shutdown ImGui before closing window
+    if (imguiInitialized) {
+        ImGui::SFML::Shutdown();
+        imguiInitialized = false;
+    }
+
     // Force close the window if it's still open
     if (window && window->isOpen()) {
         window->close();
@@ -224,6 +237,11 @@ void GameEngine::run()
         
         if (!headless) {
             sUserInput();
+
+            // Update ImGui
+            if (imguiInitialized) {
+                ImGui::SFML::Update(*window, clock.getElapsedTime());
+            }
         }
         if (!paused)
         {
@@ -262,6 +280,12 @@ void GameEngine::run()
             profilerOverlay->render(*render_target);
         }
 
+        // Render ImGui console overlay
+        if (imguiInitialized && !headless) {
+            console.render();
+            ImGui::SFML::Render(*window);
+        }
+
         // Display the frame
         if (headless) {
             headless_renderer->display();
@@ -420,6 +444,26 @@ void GameEngine::sUserInput()
     sf::Event event;
     while (window && window->pollEvent(event))
     {
+        // Process event through ImGui first
+        if (imguiInitialized) {
+            ImGui::SFML::ProcessEvent(*window, event);
+        }
+
+        // Handle grave/tilde key for console toggle (before other processing)
+        if (event.type == sf::Event::KeyPressed && event.key.code == sf::Keyboard::Grave) {
+            console.toggle();
+            continue;  // Don't pass grave key to game
+        }
+
+        // If console wants keyboard, don't pass keyboard events to game
+        if (console.wantsKeyboardInput()) {
+            // Still process non-keyboard events (mouse, window close, etc.)
+            if (event.type == sf::Event::KeyPressed || event.type == sf::Event::KeyReleased ||
+                event.type == sf::Event::TextEntered) {
+                continue;
+            }
+        }
+
         processEvent(event);
     }
 }