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docs(readme): spawn + co_await example; document .await() deadlock boundary - #90
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…undary The README's top example used xpp::fiber + .await(). Switch it to xpp::spawn + co_await — the idiomatic pairing for the poll-driven spawn executor, while .await() is the fiber pairing. Discovered while validating the example that .await() inside a spawned fn deadlocks on cross-task dependency cycles (nested park blocks the outer loop from re-checking its woken flag). Document the boundary in spawn.h and file todos/fiber-driven-spawn-await.md for a fiber-driven spawn that would make .await() universally safe.
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背景
README 顶部示例原用
xpp::fiber+.await()。换成spawn时发现：spawn是poll 驱动的执行体，它的惯用等待是
co_await（或 C++11 的.then()），而.await()是 fiber 的等待方式。改动
fiber + .await()→spawn + co_await（channel 生产/消费，sum=210）spawn.h文档：补充.await()在 spawn fn 里的跨任务依赖循环死锁边界——嵌套 park 使外层
X_RUN_ONCE无法返回检查 woken，consumer 等生产、producer等消费即死锁；推荐
.then()/co_await（waker 驱动、无嵌套栈）todos/fiber-driven-spawn-await.md：记录根治方向——spawn 的 fn 跑在 fiber 里，.await()自动走xFiberYield（不嵌套 loop），让.await()在 spawn 里普遍可用关键发现
PromiseContext::park()已有 fiber 分支（xFiberYield），但 spawn 的 fn 跑在普通栈上，
.await()走同步阻塞 + 嵌套X_RUN_ONCE。两个 spawn 任务互相 await 时形成嵌套 park 环 → 死锁。timer/I/O 场景安全（回调不嵌套 park）；依赖循环必死锁。
验证
co_await 版示例临时测试通过（
sum: 210，无死锁），临时文件已删除。