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?

Beginne in conn.go bei mu.lock in Zeile 286 und verfolge, wie sein Kontext und geschlossene Channels von den Aufrufern verwendet werden. Vergleiche das Verhalten bei der Abbruchbehandlung und beim Erwerben des Locks und bestimme anschließend, ob der gemeldete Fall eine Code- oder Dokumentationsänderung erfordert; abgeschlossen bedeutet, dass die Frage des Issues mit einer reproduzierbaren Begründung beantwortet ist.

Vom Indexierungsmodell aus dem Issue-Text verfasst.