ZDK adds event-driven service patterns

ZDK now ships constructs for event-driven services: an event bus pattern with explicit routing rules, buffered queue consumers with partial-batch failure handling, and dead-letter resources wired to alarms from the start.

Like every ZDK construct, the new patterns are thin L3 layers over standard AWS CDK resources. The defaults are visible in the source, every underlying resource stays reachable, and you can eject to plain CDK at any point without rewriting the stack.

The patterns are available in the current release on GitHub. The README includes a worked example that publishes a domain event, routes it to two consumers, and exercises the dead-letter path.