- Defined `hopgate_stream.proto` with message definitions for stream-based DTLS tunneling, including `Request`, `Response`, `StreamOpen`, `StreamData`, `StreamAck`, and `StreamClose`.
- Added `Envelope` container for top-level message encapsulation.
- Integrated Protobuf code generation into the `Makefile` using `protoc` with `protoc-gen-go`.
- Generated Go types under `internal/protocol/pb`.