WorkerService now holds eventbus.EventBus + codec.Codec instead of
*kafka.Producer / *kafka.Consumer. Receive() satisfies the
eventbus.Handler signature; Produce() goes through Codec.Serialize +
Bus.Publish.
events.Publisher likewise switches to (bus, codec) and stops
serializing via *kafka.Avrov2 directly.
The kafka package and its Avrov2/Producer/Consumer types remain for
the few non-worker call sites (tracking consumer, validate_credentials)
that haven't been migrated yet; bundled with KafkaBus.Producer() so
existing Avro framing on Kafka is preserved.
Worker boot wiring is split into cmd/* entry-point commits.
Matthew Meszaros