NwSendFailedException

Thrown by NwApi.send for an immediately-known send failure — the addressed NwConnectionId is unknown to this binding or already closed (#2455).

A plain Exception, deliberately not a kotlin.coroutines.cancellation.CancellationException, so a caller guarding the send with runCatchingCancellable sees a fabric failure rather than its own structured cancellation (the same reasoning as NwUnreachableException).

It says the frame was never handed to the transport, which is strictly narrower than "not delivered": a send that returns normally has only been handed off, and may still be lost. See NwApi.send.