Phase 4.1: Extract GridData base class from UIGrid (#252, #270, #271, #277)

Extract all grid data members and methods into GridData base class.
UIGrid now inherits from both UIDrawable (rendering) and GridData (state).

- GridData holds: grid dimensions, cell storage (flat/chunked), entities,
  spatial hash, TCOD map, FOV state, Dijkstra caches, layers, cell
  callbacks, children collection
- GridData provides: at(), syncTCODMap/Cell(), computeFOV(), isInFOV(),
  layer management (add/remove/sort/getByName), initStorage()
- UIGrid retains: texture, box, sprites, renderTexture, camera (center,
  zoom, rotation), fill_color, perspective, cell hover/click dispatch,
  all Python API static methods, render()

Fix dangling parent_grid pointers: change UIGrid* to GridData* in
GridLayer, UIGridPoint, GridChunk, ChunkManager (closes #270, closes
#271, closes #277). All 258 tests pass unchanged.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/UIGridPoint.h

@@ -16,6 +16,7 @@ static PyObject* sfColor_to_PyObject(sf::Color color);
 static sf::Color PyObject_to_sfColor(PyObject* obj);
 
 class UIGrid;
+class GridData;
 class UIEntity;
 class UIGridPoint;
 class UIGridPointState;
@@ -40,7 +41,7 @@ class UIGridPoint
 public:
     bool walkable, transparent;  // Pathfinding/FOV properties
     int grid_x, grid_y;          // Position in parent grid
-    UIGrid* parent_grid;         // Parent grid reference for TCOD sync
+    GridData* parent_grid;       // Parent grid reference for TCOD sync (#252)
     UIGridPoint();
 
     // Built-in property accessors (walkable, transparent only)