Fix gridstate heap overflows and spatial hash cleanup

Add ensureGridstate() helper that unconditionally checks gridstate size
against current grid dimensions and resizes if mismatched. Replace all
lazy-init guards (size == 0) with ensureGridstate() calls.

Previously, gridstate was only initialized when empty. When an entity
moved to a differently-sized grid, gridstate kept the old size, causing
heap buffer overflows when updateVisibility() or at() iterated using the
new grid's dimensions.

Also adds spatial_hash.remove() calls in set_grid() before removing
entities from old grids, and replaces PyObject_GetAttrString type lookup
with direct &mcrfpydef::PyUIGridType reference.

Closes #258, closes #259, closes #260, closes #261, closes #262,
closes #263, closes #274, closes #276, closes #278

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

src/UIEntity.h

@@ -73,6 +73,7 @@ public:
     ~UIEntity();
     
     // Visibility methods
+    void ensureGridstate();   // Resize gridstate to match current grid dimensions
     void updateVisibility();  // Update gridstate from current FOV
     
     // Property system for animations