0104 — `MoveCursor` stays a bare `SET position = $1` with no monotonic guard

Context. A cursor must never move backward. A GREATEST(position, $1) guard would enforce that in SQL, but it also changes what RowsAffected()==0 means.

Decision. Leave MoveCursor as a bare UPDATE cursor SET position = $1. With an ordered claim and one consumer per group, advances are strictly ascending, so monotonicity holds without a guard — and the bare UPDATE keeps the RowsAffected()==0 check meaning “unregistered group”.

Consequences. Safe only under the one-consumer-per-group model; the guard becomes necessary once concurrent advances hit a shared cursor. Superseded when the claim-from-log work replaced position with the committed waterline and MoveCursor gained the monotonic guard WHERE committed < $1.