Phase 3: Behavior system with grid.step() turn manager

- Add EntityBehavior struct with 11 behavior types: IDLE, CUSTOM,
  NOISE4/8, PATH, WAYPOINT, PATROL, LOOP, SLEEP, SEEK, FLEE.
  Each returns BehaviorOutput (MOVED/DONE/BLOCKED/NO_ACTION) without
  modifying entity position directly (closes #300)
- Add grid.step(n=1, turn_order=None) turn manager: groups entities
  by turn_order, executes behaviors, fires triggers (TARGET/DONE/BLOCKED),
  updates cell_position and spatial hash. Snapshot-based iteration for
  callback safety (closes #301)
- Entity properties: behavior_type (read-only), turn_order, move_speed,
  target_label, sight_radius. Method: set_behavior(type, waypoints,
  turns, path)
- Update ColorLayer::updatePerspective to use cell_position

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/UIGrid.h

@@ -232,6 +232,9 @@ public:
     void center_camera();  // Center on grid's middle tile
     void center_camera(float tile_x, float tile_y);  // Center on specific tile
 
+    // #301 - Turn management
+    static PyObject* py_step(PyUIGridObject* self, PyObject* args, PyObject* kwds);
+
     static PyMethodDef methods[];
     static PyGetSetDef getsetters[];
     static PyMappingMethods mpmethods;  // For grid[x, y] subscript access