refactor(chess): 重构象棋程序基础结构,使用Nullable重构核心代码

- 更新了多个文件的代码结构和类型定义，提高了代码的健壮性和可维护性
- 优化了事件处理、棋子管理和移动逻辑，为后续功能扩展打下坚实基础
- 修复了一些潜在的空指针异常问题，提高了程序的稳定性
- 修复部分bug

Scripts/Src/ChineseChess/CCBoard.cs

@@ -6,7 +6,7 @@ using Vector2I = Vector.Vector2I;
 namespace ChineseChess;
 public class CCBoard : AbstractBoard
 {
-    public event EventHandler<int> OnStepsChanged;
+    public event EventHandler<int>? OnStepsChanged;
     private int steps = 0;
     public int Steps {
         get => steps;
@@ -20,7 +20,7 @@ public class CCBoard : AbstractBoard
     public CCBoard() : base(9, 10) {
     }
 
-    public override void AddRecord(IPiece From, IPiece To, Vector2I FromPos, Vector2I ToPos) {
+    public override void AddRecord(IPiece? From, IPiece? To, Vector2I FromPos, Vector2I ToPos) {
         base.AddRecord(From, To, FromPos, ToPos);
         Steps += 1;
     }