feat(Python): establish proper inheritance hierarchy for UI types

All UIDrawable-derived Python types now properly inherit from the Drawable
base class in Python, matching the C++ inheritance structure.

Changes:
- Add Py_TPFLAGS_BASETYPE to PyDrawableType to allow inheritance
- Set tp_base = &mcrfpydef::PyDrawableType for all UI types
- Add PyDrawable.h include to UI type headers
- Rename _Drawable to Drawable and update error message

This enables proper Python inheritance: Frame, Caption, Sprite, Grid,
and Entity all inherit from Drawable, allowing shared functionality
and isinstance() checks.

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

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

src/PyDrawable.cpp

@@ -154,14 +154,14 @@ static PyMethodDef PyDrawable_methods[] = {
 // Type initialization
 static int PyDrawable_init(PyDrawableObject* self, PyObject* args, PyObject* kwds)
 {
-    PyErr_SetString(PyExc_TypeError, "_Drawable is an abstract base class and cannot be instantiated directly");
+    PyErr_SetString(PyExc_TypeError, "Drawable is an abstract base class and cannot be instantiated directly");
     return -1;
 }
 
 namespace mcrfpydef {
     PyTypeObject PyDrawableType = {
         .ob_base = {.ob_base = {.ob_refcnt = 1, .ob_type = NULL}, .ob_size = 0},
-        .tp_name = "mcrfpy._Drawable",
+        .tp_name = "mcrfpy.Drawable",
         .tp_basicsize = sizeof(PyDrawableObject),
         .tp_itemsize = 0,
         .tp_dealloc = (destructor)[](PyObject* self) {
@@ -169,7 +169,7 @@ namespace mcrfpydef {
             obj->data.reset();
             Py_TYPE(self)->tp_free(self);
         },
-        .tp_flags = Py_TPFLAGS_DEFAULT, // | Py_TPFLAGS_BASETYPE,
+        .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
         .tp_doc = PyDoc_STR("Base class for all drawable UI elements"),
         .tp_methods = PyDrawable_methods,
         .tp_getset = PyDrawable_getsetters,