backoff: flat NakWithDelay redelivery policy with Term-on-final-delivery

Lift the COR-762 fix from mirror-pipeline's fanoutengine, retargeted at jetstream.Msg: Policy applies a flat NakWithDelay envelope to transient failures, and with TermOnExhaustion set the final delivery Terms instead — on a work-queue stream a Nak the broker drops at MaxDeliver would otherwise orphan the message un-acked until StreamMaxAge, pinning the oldest-pending-age gauge.

Disposition only: DLQ capture stays with the caller, gated on the exported IsFinalDelivery (NumDelivered is exported for logs/attrs). NakOrTerm returns the chosen Outcome for the caller's metrics plus any disposition error for its logs.