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.cpp

@@ -9,6 +9,7 @@
 #include "PyWindow.h"
 #include "PySceneObject.h"
 #include "GameEngine.h"
+#include "ImGuiConsole.h"
 #include "UI.h"
 #include "UILine.h"
 #include "UICircle.h"
@@ -202,6 +203,16 @@ static PyMethodDef mcrfpyMethods[] = {
          MCRF_RETURNS("dict: Performance data with keys: frame_time (last frame duration in seconds), avg_frame_time (average frame time), fps (frames per second), draw_calls (number of draw calls), ui_elements (total UI element count), visible_elements (visible element count), current_frame (frame counter), runtime (total runtime in seconds)")
      )},
 
+    {"setDevConsole", McRFPy_API::_setDevConsole, METH_VARARGS,
+     MCRF_FUNCTION(setDevConsole,
+         MCRF_SIG("(enabled: bool)", "None"),
+         MCRF_DESC("Enable or disable the developer console overlay."),
+         MCRF_ARGS_START
+         MCRF_ARG("enabled", "True to enable the console (default), False to disable")
+         MCRF_RETURNS("None")
+         MCRF_NOTE("When disabled, the grave/tilde key will not open the console. Use this to ship games without debug features.")
+     )},
+
     {NULL, NULL, 0, NULL}
 };
 
@@ -1195,6 +1206,16 @@ PyObject* McRFPy_API::_getMetrics(PyObject* self, PyObject* args) {
     return dict;
 }
 
+PyObject* McRFPy_API::_setDevConsole(PyObject* self, PyObject* args) {
+    int enabled;
+    if (!PyArg_ParseTuple(args, "p", &enabled)) {  // "p" for boolean predicate
+        return NULL;
+    }
+
+    ImGuiConsole::setEnabled(enabled);
+    Py_RETURN_NONE;
+}
+
 // Exception handling implementation
 void McRFPy_API::signalPythonException() {
     // Check if we should exit on exception (consult config via game)