Radio networking module skeleton (addresses #2)

[Bennethxyz]

This PR adds a minimal radio networking module to exo.

What

• RadioTransport abstraction with an in-memory LoopbackHub/LoopbackRadioTransport
• RadioServer: handles a small JSON protocol to proxy Node operations over radio
• RadioPeerHandle: sends prompts/tensors/results via RadioTransport
• RadioDiscovery: loopback-based beaconing; serves as a reference for real radios

Why

• Addresses [#2] Radio Networking Module bounty. This provides the interfaces and a reference implementation to plug in real radio transports (LoRa, AX.25/KISS, etc.) without changing Node.

Notes

• Backwards compatible: doesn’t alter existing gRPC/UDP/Tailscale modules
• Next steps: implement a concrete LoRa/Serial transport and simple framing with retries/acks.