Observer

A listener subscribed to a run's thread during admission - before the run's claim commits, with no replay - so it receives every durable event of the run from its claim, and every live event, which a subscription made after start returns can miss. capacity is the subscription's queue size, at least 2, as for GraphRuntime.subscribe. The subscription is thread-scoped like any other: it keeps delivering later runs until cancelled, through RunHandle.observation. A refused admission abandons it, and its listener is never called.