0124 — Fan-out does not need a monotonic cursor guard; `MoveCursor` stays `SET position = $1`

Context. An earlier forecast said a GREATEST(position, $1) guard would be needed “once fan-out puts concurrent advances on a shared cursor.” That forecast was wrong about where the risk lives: fan-out is different groups on different cursor rows, one consumer each.

Decision. Keep MoveCursor as the bare SET position = $1. Advances on any one cursor are still strictly ascending under one-consumer-per-group, so monotonicity holds without a guard. Concurrent advances on a shared cursor only appear when multiple workers compete within a group.

Consequences. The guard is deferred to the within-group claim-from-log work, where it actually belongs — and it landed there, superseding this: MoveCursor became committed = $1 WHERE committed < $1. Until then, RowsAffected()==0 keeps its single meaning of “unregistered group”.