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>

test/programs/40_multi_ret.l

@@ -0,0 +1,10 @@
+fn div_mod(a: i64, b: i64) -> (i64, i64) {
+    return (a / b, a % b);
+}
+
+fn main() -> i64 {
+    let result = div_mod(10, 3);
+    print_i64(result._0);  // 3
+    print_i64(result._1);  // 1
+    return 0;
+}