Grid/GridView API unification: mcrfpy.Grid now returns GridView, closes #252

mcrfpy.Grid() now creates a GridView that internally owns a GridData (UIGrid).
The old UIGrid type is renamed to _GridData (internal). Attribute access on Grid
delegates to the underlying UIGrid via tp_getattro/tp_setattro, so all existing
Grid properties (grid_w, grid_h, entities, cells, layers, etc.) work transparently.

Key changes:
- GridView init has two modes: factory (Grid(grid_size=...)) and explicit view
  (Grid(grid=existing_grid, ...)) for future multi-view support
- Entity.grid getter returns GridView wrapper via owning_view back-reference
- Entity.grid setter accepts GridView objects
- GridLayer set_grid handles GridView (extracts underlying UIGrid)
- UIDrawable::removeFromParent handles UIGRIDVIEW type correctly
- UIFrame children init accepts GridView objects
- Animation system supports GridView (center, zoom, shader.* properties)
- PythonObjectCache registration preserves subclass identity
- All 263 tests pass (100%)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/UIFrame.cpp

@@ -745,7 +745,8 @@ int UIFrame::init(PyUIFrameObject* self, PyObject* args, PyObject* kwds)
             if (!PyObject_IsInstance(child, (PyObject*)&mcrfpydef::PyUIFrameType) &&
                 !PyObject_IsInstance(child, (PyObject*)&mcrfpydef::PyUICaptionType) &&
                 !PyObject_IsInstance(child, (PyObject*)&mcrfpydef::PyUISpriteType) &&
-                !PyObject_IsInstance(child, (PyObject*)&mcrfpydef::PyUIGridType)) {
+                !PyObject_IsInstance(child, (PyObject*)&mcrfpydef::PyUIGridType) &&
+                !PyObject_IsInstance(child, (PyObject*)&mcrfpydef::PyUIGridViewType)) {
                 Py_DECREF(child);
                 PyErr_SetString(PyExc_TypeError, "children must contain only Frame, Caption, Sprite, or Grid objects");
                 return -1;
@@ -759,6 +760,8 @@ int UIFrame::init(PyUIFrameObject* self, PyObject* args, PyObject* kwds)
                 drawable = ((PyUICaptionObject*)child)->data;
             } else if (PyObject_IsInstance(child, (PyObject*)&mcrfpydef::PyUISpriteType)) {
                 drawable = ((PyUISpriteObject*)child)->data;
+            } else if (PyObject_IsInstance(child, (PyObject*)&mcrfpydef::PyUIGridViewType)) {
+                drawable = ((PyUIGridViewObject*)child)->data;
             } else if (PyObject_IsInstance(child, (PyObject*)&mcrfpydef::PyUIGridType)) {
                 drawable = ((PyUIGridObject*)child)->data;
             }