feat: for-of 迭代器栈条目化：迭代器与最近 next 结果配对存放

单一 last_for_of_result 槽被嵌套迭代覆盖，CLOSE 判定本迭代器是否
自然 done 时误读他者结果，导致数组解构/spread/嵌套 for-of 提前退出
漏调或误调 return()。改为栈条目 (iterator, last_result) 配对：

- IterState.for_of_iters 元素改 ForOfEntry，删除 last_for_of_result
- INIT 压条目，DONE 写本条目，NEXT/CLOSE 读本条目结果判 done
- GC mark/rewrite、inline save/restore、挂起帧快照同步配对遍历
- for-await-of 数组解构提前 break 现正确触发 return()（任务 40 样例）

crates/oxide_vm/src/session_gc.rs

@@ -839,8 +839,10 @@ mod tests {
         vm.regs[1] = JsValue::from_js_object(child_session);
         vm.exception_value = Some(JsValue::from_js_object(root_session));
         vm.pending_exception = Some(JsValue::from_js_object(child_session));
-        vm.iters.for_of_iters.push(JsValue::from_js_object(child_session));
-        vm.iters.last_for_of_result = JsValue::from_js_object(root_session);
+        vm.iters.for_of_iters.push(crate::vm_state::ForOfEntry {
+            iterator: JsValue::from_js_object(child_session),
+            last_result: JsValue::from_js_object(root_session),
+        });
 
         let mut roots = Vec::new();
         vm.for_each_root(|v| roots.push(v));