0125 — A `consumerFunc` error stops the whole poll loop; per-group retry and DLQ are deferred

Context. The cursor model has no per-row failure resolution — a single integer cannot record that one message failed while its neighbours succeeded. Fan-out adds nothing to change that.

Decision. Keep failure semantics unchanged: a consumerFunc error returns up through Claim and Process and stops the poll loop. The fan-out lab runs at fail-rate=0 to keep the focus on independent positions.

Consequences. A failing message head-of-line blocks its group until the operator intervenes. This is a deliberate deferral, not a gap: per-group retry and dead-lettering are the later deliveries work. Trying to solve failure inside the bare cursor model would reintroduce per-row state the split just removed.