Replaces Python+HAProxy gateway stack with a single Go binary:
- pkg/pairing: handshake with lthn.io (pair + heartbeat)
- pkg/meter: per-session bandwidth metering
- cmd/gateway: main binary with admin API on :8880
- Uses core/api for HTTP, pairs via /v1/gateway/* endpoints
Eliminates: Python lvpn, HAProxy config templating, legacy session management.
Keeps: WireGuard (managed via go-process when ready).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
