The prompt response callback now sends an AcpUpdate::TurnDone marker
through the same unbounded channel as notifications instead of
signaling the done oneshot directly. Since notifications and the
prompt response both run in the ACP dispatch loop in wire order,
TurnDone lands after every notification chunk for the turn.
The event pump (next commit) will fire the done oneshot only after
persisting all preceding notifications, closing the race where
finish_queued_acp_turn ran before the transcript was complete.
Closes#259
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
