fix: prevent segfault when closing window via X button

- Add cleanup() method to GameEngine to clear Python references before destruction
- Clear timers and McRFPy_API references in proper order
- Call cleanup() at end of run loop and in destructor
- Ensure cleanup is only called once per GameEngine instance

Also includes:
- Fix audio ::stop() calls (already in place, OpenAL warning is benign)
- Add Caption support for x, y keywords (e.g. Caption("text", x=5, y=10))
- Refactor UIDrawable_methods.h into UIBase.h for better organization
- Move UIEntity-specific implementations to UIEntityPyMethods.h

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/UIEntityPyMethods.h

@@ -0,0 +1,48 @@
+#pragma once
+#include "UIEntity.h"
+#include "UIBase.h"
+
+// UIEntity-specific property implementations
+// These delegate to the wrapped sprite member
+
+// Visible property
+static PyObject* UIEntity_get_visible(PyUIEntityObject* self, void* closure)
+{
+    return PyBool_FromLong(self->data->sprite.visible);
+}
+
+static int UIEntity_set_visible(PyUIEntityObject* self, PyObject* value, void* closure)
+{
+    if (!PyBool_Check(value)) {
+        PyErr_SetString(PyExc_TypeError, "visible must be a boolean");
+        return -1;
+    }
+    self->data->sprite.visible = PyObject_IsTrue(value);
+    return 0;
+}
+
+// Opacity property
+static PyObject* UIEntity_get_opacity(PyUIEntityObject* self, void* closure)
+{
+    return PyFloat_FromDouble(self->data->sprite.opacity);
+}
+
+static int UIEntity_set_opacity(PyUIEntityObject* self, PyObject* value, void* closure)
+{
+    float opacity;
+    if (PyFloat_Check(value)) {
+        opacity = PyFloat_AsDouble(value);
+    } else if (PyLong_Check(value)) {
+        opacity = PyLong_AsDouble(value);
+    } else {
+        PyErr_SetString(PyExc_TypeError, "opacity must be a number");
+        return -1;
+    }
+    
+    // Clamp to valid range
+    if (opacity < 0.0f) opacity = 0.0f;
+    if (opacity > 1.0f) opacity = 1.0f;
+    
+    self->data->sprite.opacity = opacity;
+    return 0;
+}