feat: 多返回值 fn f(...) -> (T, U) { return (e1, e2); }

Parser: -> (T1,T2) 解析为隐式 struct __ret_funcname, return (e1,e2) 去糖为 struct init
返回的 struct 可访问字段 _0, _1, ... 无 sema/codegen 改动

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ast/ast.h

@@ -71,7 +71,8 @@ struct AstNode {
         struct { const char* name; struct AstNode** params; size_t param_count;
                  TypeKind return_type; const char* return_struct_type_name;
                  struct AstNode* body; bool is_pub;
-                 const char** type_params; size_t type_param_count; } function;
+                 const char** type_params; size_t type_param_count;
+                 TypeKind* multi_ret_types; const char** multi_ret_snames; size_t multi_ret_count; } function;
         // AST_PARAMETER (也用作结构体字段: name + type)
         struct { const char* name; TypeKind type; const char* struct_type_name; } parameter;
         // AST_BLOCK