Implements DeliveryInterestTracker (consumer.go hasDeliveryInterest /
deleteNotActive) with thread-safe subscribe/unsubscribe counting, a
configurable inactivity timeout for ephemeral consumer auto-deletion, and
10 covering tests. Also adds SlopwatchSuppress to a pre-existing
Task.Delay in MaxDeliveriesTests.cs that was already testing time-based expiry.
