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/GridLayers.h

@@ -5,6 +5,7 @@
 #include <SFML/Graphics.hpp>
 #include <memory>
 #include <vector>
+#include <string>
 
 // Forward declarations
 class UIGrid;
@@ -23,6 +24,7 @@ enum class GridLayerType {
 class GridLayer {
 public:
     GridLayerType type;
+    std::string name;      // #150 - Layer name for GridPoint property access
     int z_index;           // Negative = below entities, >= 0 = above entities
     int grid_x, grid_y;    // Dimensions
     UIGrid* parent_grid;   // Parent grid reference