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/UIGrid.h

@@ -8,6 +8,7 @@
 
 #include "PyCallable.h"
 #include "PyTexture.h"
+#include "PyDrawable.h"
 #include "PyColor.h"
 #include "PyVector.h"
 #include "PyFont.h"
@@ -39,6 +40,7 @@ public:
     sf::FloatRect get_bounds() const override;
     void move(float dx, float dy) override;
     void resize(float w, float h) override;
+    void onPositionChanged() override;
 
     int grid_x, grid_y;
     //int grid_size; // grid sizes are implied by IndexTexture now
@@ -153,7 +155,7 @@ namespace mcrfpydef {
         .tp_methods = UIGrid_all_methods,
         //.tp_members = UIGrid::members,
         .tp_getset = UIGrid::getsetters,
-        //.tp_base = NULL,
+        .tp_base = &mcrfpydef::PyDrawableType,
         .tp_init = (initproc)UIGrid::init,
         .tp_new = [](PyTypeObject* type, PyObject* args, PyObject* kwds) -> PyObject*
         {