Constant ARRANGEMENT_SIZES_FRESHNESS_MARGIN 

const ARRANGEMENT_SIZES_FRESHNESS_MARGIN: Duration;

How long an introspection subscribe must have been delivering data before the arrangement sizes snapshot trusts its replica’s rows.

See Coordinator::fresh_introspection_replicas for why a margin is needed. 10s comfortably covers the collection manager’s ~1s write batching plus the oracle read timestamp trailing the wall clock.