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/McRFPy_API.cpp

@@ -15,6 +15,7 @@
 #include "UILine.h"
 #include "UICircle.h"
 #include "UIArc.h"
+#include "GridLayers.h"
 #include "Resources.h"
 #include "PyScene.h"
 #include <filesystem>
@@ -303,6 +304,9 @@ PyObject* PyInit_mcrfpy()
         /*game map & perspective data*/
         &PyUIGridPointType, &PyUIGridPointStateType,
 
+        /*grid layers (#147)*/
+        &PyColorLayerType, &PyTileLayerType,
+
         /*collections & iterators*/
         &PyUICollectionType, &PyUICollectionIterType,
         &PyUIEntityCollectionType, &PyUIEntityCollectionIterType,