更新 HEICFile 类以支持新特性，添加图像尺寸获取功能，重构解析逻辑，增强代码可读性和健壮性

base.py

@@ -12,4 +12,8 @@ class Box:
         self.children: List['Box'] = [] # Initially empty
 
     def __repr__(self) -> str:
-        return f"<Box '{self.type}' size={self.size} offset={self.offset}>"
+        return f"<Box '{self.type}' size={self.size} offset={self.offset}>"
+        
+    def _post_parse_initialization(self):
+        """Called by the parser after children have been assigned."""
+        pass