When retain_recent_legal_suffix hits the else branch (tail has no user
messages), it takes a non-contiguous slice from the middle of the session.
enforce_file_cap incorrectly assumed dropped messages were always a prefix
(before[:dropped_count]), causing user messages to be both archived and
retained, and some messages to silently disappear.
Fix by having retain_recent_legal_suffix return the actual dropped message
list using identity-based diff, so enforce_file_cap no longer needs to
guess which messages were removed.
72fb642ef7