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>

src/UIGrid.h

@@ -95,11 +95,15 @@ public:
     std::vector<std::shared_ptr<GridLayer>> layers;
     bool layers_need_sort = true;  // Dirty flag for z_index sorting
 
-    // Layer management
-    std::shared_ptr<ColorLayer> addColorLayer(int z_index);
-    std::shared_ptr<TileLayer> addTileLayer(int z_index, std::shared_ptr<PyTexture> texture = nullptr);
+    // Layer management (#150 - extended with names)
+    std::shared_ptr<ColorLayer> addColorLayer(int z_index, const std::string& name = "");
+    std::shared_ptr<TileLayer> addTileLayer(int z_index, std::shared_ptr<PyTexture> texture = nullptr, const std::string& name = "");
     void removeLayer(std::shared_ptr<GridLayer> layer);
     void sortLayers();
+    std::shared_ptr<GridLayer> getLayerByName(const std::string& name);
+
+    // #150 - Protected layer names (reserved for GridPoint properties)
+    static bool isProtectedLayerName(const std::string& name);
 
     // Background rendering
     sf::Color fill_color;