- Changed `go_package` option in `hopgate_stream.proto` to `internal/protocol/pb;pb`.
- Regenerated `hopgate_stream.pb.go` with updated package path to align with new structure.
- Added `protocol.md` documenting the gRPC-based HTTP tunneling protocol.
- Introduced gRPC-based tunnel design for bi-directional communication, replacing legacy DTLS transport.
- Added `HopGateTunnel` gRPC service with client and server logic for `OpenTunnel` stream handling.
- Updated client to use gRPC tunnel exclusively, including experimental entry point for stream-based HTTP proxying.
- Removed DTLS-specific client, server, and related dependencies (`pion/dtls`).
- Adjusted `cmd/server` to route gRPC and HTTP/HTTPS traffic dynamically on shared ports.