A caller-directed Send that failed after a fragment left called
EndUnsendablePeer on the caller's thread, removing from _peers and raising
PeerDisconnected while the polling thread could be changing _peers too.
Send now ends the session and sends the goodbye under the peer's
SessionGate, and queues the peer; TryReceiveOnce and PollResends retire
queued peers before anything else. A peer a new Connect already replaced
is skipped, so a stale peer object cannot remove the peer that replaced it.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>