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/UIGridPoint.h

@@ -52,6 +52,10 @@ public:
     static PyObject* get_bool_member(PyUIGridPointObject* self, void* closure);
     static int set_color(PyUIGridPointObject* self, PyObject* value, void* closure);
     static PyObject* repr(PyUIGridPointObject* self);
+
+    // #150 - Dynamic property access for named layers
+    static PyObject* getattro(PyUIGridPointObject* self, PyObject* name);
+    static int setattro(PyUIGridPointObject* self, PyObject* name, PyObject* value);
 };
 
 // UIGridPointState - entity-specific info for each cell
@@ -73,6 +77,9 @@ namespace mcrfpydef {
         .tp_basicsize = sizeof(PyUIGridPointObject),
         .tp_itemsize = 0,
         .tp_repr = (reprfunc)UIGridPoint::repr,
+        // #150 - Dynamic attribute access for named layers
+        .tp_getattro = (getattrofunc)UIGridPoint::getattro,
+        .tp_setattro = (setattrofunc)UIGridPoint::setattro,
         .tp_flags = Py_TPFLAGS_DEFAULT,
         .tp_doc = "UIGridPoint object",
         .tp_getset = UIGridPoint::getsetters,