In-process Event Bus

Design a single-process publish/subscribe library. Listeners can unsubscribe or add subscriptions while an event is being handled. Slow or throwing listeners need documented behavior.

• Return stable subscription tokens and support idempotent unsubscribe.
• Define which subscribers receive an in-flight publication.
• Isolate listener errors and specify dispatch order.
• Bound asynchronous dispatch if an async adapter is provided.