sync_turn only knew the author through the on_turn_start stash. The memory
manager now passes turn_author and scope with the turn, and a caller
that skips on_turn_start left the stash empty or stale.
sync_turn takes both keywords and reads the author from turn_author first.
The stash stays as the fallback for callers that never pass it.
resolve_author_peer_id compared the author against the primary runtime id
only. A transport that names the participant by the alt id (Telegram
username instead of UID) got a second peer for the same person. Either
runtime id now counts as the session's own participant.
20b117ad3b