refactor: Remove layer-related GridPoint properties, fix layer z-index

- Remove color, color_overlay, tilesprite, tile_overlay, uisprite from
  UIGridPoint - these are now accessed through named layers
- Keep only walkable and transparent as protected GridPoint properties
- Update isProtectedLayerName() to only protect walkable/transparent
- Fix default layer z-index to -1 (below entities) instead of 0
- Remove dead rendering code from GridChunk (layers handle rendering)
- Update cos_level.py demo to use explicit layer definitions
- Update UITestScene.cpp to use layer API instead of GridPoint properties

Part of #150 - Grid layer system migration

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

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

src/scripts/cos_level.py

@@ -105,7 +105,9 @@ class Level:
         self.height = height
         #self.graph = [(0, 0, width, height)]
         self.graph = RoomGraph( (0, 0, width, height) )
-        self.grid = mcrfpy.Grid(grid_size=(width, height), texture=t, pos=(10, 5), size=(1014, 700))
+        # #150 - Create grid with explicit layers for color and tilesprite
+        self.grid = mcrfpy.Grid(grid_size=(width, height), texture=t, pos=(10, 5), size=(1014, 700),
+                                layers={"color": "color", "tilesprite": "tile"})
         self.highlighted = -1 #debug view feature
         self.walled_rooms = [] # for tracking "hallway rooms" vs "walled rooms"