feat(kerykeion): Meshtastic protobuf and connection layer

[forkwright]

From Phase 03 PLAN.md, REQ-01 + REQ-02 + REQ-03.

Clean-room Meshtastic protocol implementation in kerykeion crate:

• Protobuf definitions from .proto files via prost-build (not wrapping official crate)
• Connection layer: serial (USB), TCP, BLE transports
• Async reconnection with exponential backoff
• AES-CTR encryption: 16-byte nonce construction, channel PSK resolution, default key expansion
• RAK2245 gateway support
• Signal production to broadcast channel

Groups REQ-01 (protobuf), REQ-02 (connections), REQ-03 (encryption) as they form the core protocol stack.

[forkwright]

Closing — fully delivered across PRs #52 (protobuf codegen), #56 (serial/TCP transport + AES-CTR encryption), #62 (gateway bridge), #68 (signal production).