Codec interface (Serialize / Deserialize / Name) lets payload encoding
decouple from the transport choice. Two implementations:
AvroCodec - wraps the existing kafka.Avrov2 Schema Registry client
via NewAvroFromClient. Preserves identical wire format
for production deployments already on Kafka + SR.
JSONCodec - encoding/json based. No external dependency, suitable
for self-hosters who don't want a Schema Registry.
Factory FromEnv selects via CODEC_PROVIDER (default avro).
Once codec.Codec is in the worker boot and publisher, self-hosters can
pick EVENTBUS_PROVIDER=nats CODEC_PROVIDER=json for a Schema-Registry-
free deployment.
11 tests cover JSON round-trip, nil guards, factory paths, and Avro
interface conformance.
