fix: make the type of obj immutable

logic/Preparation/Interface/IGameObj.cs

@@ -4,7 +4,7 @@ namespace Preparation.Interface
 {
     public interface IGameObj
     {
-        public GameObjType Type { get; set; }
+        public GameObjType Type { get; }
         public long ID { get; }
         public XYPosition Position { get; }  // if Square, Pos equals the center
         public double FacingDirection { get; }