feat: Migrate Grid to user-driven layer rendering (closes #150)

- Add `layers` dict parameter to Grid constructor for explicit layer definitions
  - `layers={"ground": "color", "terrain": "tile"}` creates named layers
  - `layers={}` creates empty grid (entities + pathfinding only)
  - Default creates single TileLayer named "tilesprite" for backward compat

- Implement dynamic GridPoint property access via layer names
  - `grid.at(x,y).layer_name = value` routes to corresponding layer
  - Protected names (walkable, transparent, etc.) still use GridPoint

- Remove base layer rendering from UIGrid::render()
  - Layers are now the sole source of grid rendering
  - Old chunk_manager remains for GridPoint data access
  - FOV overlay unchanged

- Update test to use explicit `layers={}` parameter

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

tests/regression/issue_147_grid_layers.py

@@ -21,13 +21,13 @@ def run_test(runtime):
     ui = mcrfpy.sceneUI("test")
     texture = mcrfpy.Texture("assets/kenney_ice.png", 16, 16)
 
-    # Create grid
-    grid = mcrfpy.Grid(pos=(50, 50), size=(400, 300), grid_size=(20, 15), texture=texture)
+    # Create grid with explicit empty layers (#150 migration)
+    grid = mcrfpy.Grid(pos=(50, 50), size=(400, 300), grid_size=(20, 15), texture=texture, layers={})
     ui.append(grid)
 
     print("\n--- Test 1: Initial state (no layers) ---")
     if len(grid.layers) == 0:
-        print("  PASS: Grid starts with no layers")
+        print("  PASS: Grid starts with no layers (layers={})")
     else:
         print(f"  FAIL: Expected 0 layers, got {len(grid.layers)}")
         sys.exit(1)