BREAKING CHANGE: All NATS message serialization now uses Protocol Buffers.
- Added proto/messages/v1/messages.proto with 22 message types
- Generated Go code at gen/messagespb/
- messages/ package now exports type aliases to proto types
- natsutil.Publish/Request/Decode use proto.Marshal/Unmarshal
- Removed legacy MessageHandler, OnMessage, wrapMapHandler
- TypedMessageHandler now returns (proto.Message, error)
- EffectiveQuery is now a free function: messages.EffectiveQuery(req)
- Removed msgpack dependency entirely
