fix(ast): parse_expression 返回注解改为 Expression | t.CPtr

- parse_expression 实际返回 Expression 包装节点（body 字段为实际表达式），
  原注解 AST | t.CPtr 类型不精确——调用方需强转才能访问 body
- 源头修正返回类型，调用方直接 expr_node.body 即可

includes/ast/__init__.vp

@@ -96,7 +96,7 @@ def parse(src: str, pool: memhub.MemManager | t.CPtr) -> AST | t.CPtr:
     return tree
 
 
-def parse_expression(src: str, pool: memhub.MemManager | t.CPtr) -> AST | t.CPtr:
+def parse_expression(src: str, pool: memhub.MemManager | t.CPtr) -> Expression | t.CPtr:
     """Parse a single expression, return the Expression node.
 
     Parameters:
@@ -104,7 +104,7 @@ def parse_expression(src: str, pool: memhub.MemManager | t.CPtr) -> AST | t.CPtr
         pool: memhub.MemManager instance, to allocate AST nodes
 
     Returns:
-        Expression node, or None
+        Expression node (包装节点，body 字段为实际表达式), or None
     """
     if src is None or pool is None:
         return None