Busy steer (the busy-mode and priority paths, and /steer) pushes a new message
into the running turn, which then answers it without a turn of its own.
Redirect already folded the message's reply_expected into the turn; steer
did not, so an unaddressed opener steered by an @mention could still end
on a bare silence marker. All of them now go through _fold_into_running_turn.
Also: stale comments on MessageEvent and the queued-terminal silence
verdict, and a crash-recovery case for an unaddressed row.