feat: Add dynamic layer system for Grid (closes #147)

Implements ColorLayer and TileLayer classes with z_index ordering:
- ColorLayer: stores RGBA color per cell for overlays, fog of war, etc.
- TileLayer: stores sprite index per cell with optional texture
- z_index < 0: renders below entities
- z_index >= 0: renders above entities

Python API:
- grid.add_layer(type, z_index, texture) - create layer
- grid.remove_layer(layer) - remove layer
- grid.layers - list of layers sorted by z_index
- grid.layer(z_index) - get layer by z_index
- layer.at(x,y) / layer.set(x,y,value) - cell access
- layer.fill(value) - fill entire layer

Layers are allocated separately from UIGridPoint, reducing memory
for grids that don't need all features. Base grid retains walkable/
transparent arrays for TCOD pathfinding.

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

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

src/UIGrid.h

@@ -20,6 +20,7 @@
 #include "UIEntity.h"
 #include "UIDrawable.h"
 #include "UIBase.h"
+#include "GridLayers.h"
 
 class UIGrid: public UIDrawable
 {
@@ -81,6 +82,16 @@ public:
     std::shared_ptr<std::vector<std::shared_ptr<UIDrawable>>> children;
     bool children_need_sort = true;  // Dirty flag for z_index sorting
 
+    // Dynamic layer system (#147)
+    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);
+    void removeLayer(std::shared_ptr<GridLayer> layer);
+    void sortLayers();
+
     // Background rendering
     sf::Color fill_color;
 
@@ -147,7 +158,12 @@ public:
     static PyObject* get_on_cell_click(PyUIGridObject* self, void* closure);
     static int set_on_cell_click(PyUIGridObject* self, PyObject* value, void* closure);
     static PyObject* get_hovered_cell(PyUIGridObject* self, void* closure);
-    
+
+    // #147 - Layer system Python API
+    static PyObject* py_add_layer(PyUIGridObject* self, PyObject* args, PyObject* kwds);
+    static PyObject* py_remove_layer(PyUIGridObject* self, PyObject* args);
+    static PyObject* get_layers(PyUIGridObject* self, void* closure);
+    static PyObject* py_layer(PyUIGridObject* self, PyObject* args);
 };
 
 typedef struct {