Bug: for event in build.build_events() would deadlock because:

1. recv() blocks waiting for channel disconnect
2. Channel disconnect only happens when senders are dropped
3. Senders are only dropped in join() called from wait()
4. wait() is called after the for loop exits — but the loop never exits

Fix: Added explicit close() method to Broadcaster that drops all senders when the BES threadfinishes, triggering channel disconnect regardless of when wait() is called.

With this change, now late subscriptions via .build_events() get events replayed, the remaining build event sinks continue operate without a replayer.