Add IsTransientSubscribeErr for subscribe/create retry paths

The fetch predicate deliberately excludes ErrTimeout and context.DeadlineExceeded (idle poll / wedged server), which left every caller retrying subscription setup to classify those — plus rollout ordering's ErrStreamNotFound — by hand. Two services share these consumer loops; when the hand-rolled sets drift, the alert noise this package exists to remove comes back. Hold the subscribe-path set here, next to the fetch-path set it extends.