mu.lock: after acquiring m.ch, why does the re-check only look at closed and not ctx?

Hi, I was looking at [mu.lock](https://github.com/coder/websocket/blob/master/conn.go#L286) and had a question about this part:

```go
func (m *mu) lock(ctx context.Context) error {
select {
case <-m.c.closed:
return net.ErrClosed
case <-ctx.Done():
return fmt.Errorf("failed to acquire lock: %w", ctx.Err())
case m.ch <- struct{}{}:
// To make sure the connection is certainly alive.
// As it's possible the send on m.ch was selected
// over the receive on closed.
select {
case <-m.c.closed:
// Make sure to release.
m.unlock()
return net.ErrClosed
default:
}
return nil
}
}
```

After acquiring the lock, `closed` is checked again in case both branches were ready.

Why isn't `ctx.Done()` checked here as well?

Could `ctx` be canceled right after `m.ch` is selected, causing `lock` to return `nil` while holding the lock with an already-canceled context?

从 conn.go 第 286 行附近的 mu.lock 开始，追踪其 context 和已关闭 channel 是如何被调用方使用的。比较取消和获取 lock 的行为，然后确定报告的情况是否需要修改代码或文档；完成的标准是通过可复现的依据回答 issue 的问题。

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