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/GridData.h

@@ -26,6 +26,7 @@
 class DijkstraMap;
 class UIEntity;
 class UIDrawable;
+class UIGridView;
 class PyTexture;
 
 class GridData {
@@ -130,6 +131,11 @@ public:
     std::shared_ptr<std::vector<std::shared_ptr<UIDrawable>>> children;
     bool children_need_sort = true;
 
+    // =========================================================================
+    // #252 - Owning GridView back-reference (for Entity.grid → GridView lookup)
+    // =========================================================================
+    std::weak_ptr<UIGridView> owning_view;
+
 protected:
     // Initialize grid storage (flat or chunked) and TCOD map
     void initStorage(int gx, int gy, GridData* parent_ref);