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>

CMakeLists.txt

@@ -17,16 +17,36 @@ include_directories(${CMAKE_SOURCE_DIR}/deps/libtcod)
 include_directories(${CMAKE_SOURCE_DIR}/deps/cpython)
 include_directories(${CMAKE_SOURCE_DIR}/deps/Python)
 
+# ImGui and ImGui-SFML include directories
+include_directories(${CMAKE_SOURCE_DIR}/modules/imgui)
+include_directories(${CMAKE_SOURCE_DIR}/modules/imgui-sfml)
+
+# ImGui source files
+set(IMGUI_SOURCES
+    ${CMAKE_SOURCE_DIR}/modules/imgui/imgui.cpp
+    ${CMAKE_SOURCE_DIR}/modules/imgui/imgui_draw.cpp
+    ${CMAKE_SOURCE_DIR}/modules/imgui/imgui_tables.cpp
+    ${CMAKE_SOURCE_DIR}/modules/imgui/imgui_widgets.cpp
+    ${CMAKE_SOURCE_DIR}/modules/imgui-sfml/imgui-SFML.cpp
+)
+
 # Collect all the source files
 file(GLOB_RECURSE SOURCES "src/*.cpp")
 
+# Add ImGui sources to the build
+list(APPEND SOURCES ${IMGUI_SOURCES})
+
+# Find OpenGL (required by ImGui-SFML)
+find_package(OpenGL REQUIRED)
+
 # Create a list of libraries to link against
-set(LINK_LIBS 
-    sfml-graphics 
-    sfml-window 
-    sfml-system 
-    sfml-audio 
-    tcod)
+set(LINK_LIBS
+    sfml-graphics
+    sfml-window
+    sfml-system
+    sfml-audio
+    tcod
+    OpenGL::GL)
 
 # On Windows, add any additional libs and include directories
 if(WIN32)