hostd: TCP/WebSocket listener for companion protocol

[dougborg]

Summary

Add a TCP and/or WebSocket listener to hostd so companion apps can connect over the network, not just via stdin/stdout piping.

Context

Currently hostd outputs NDJSON to stdout and reads commands from stdin. This works for local piping but isn't practical for GUI companion apps or remote monitoring.

Implementation

• Add --listen <addr:port> CLI flag (e.g., --listen 0.0.0.0:4200)
• TCP listener using tokio::net::TcpListener
• Each connection gets a clone of the output stream (broadcast channel) and can send commands
• Optional WebSocket upgrade using tokio-tungstenite for browser-based companions
• Same NDJSON protocol over TCP/WebSocket as over stdio
• Multiple simultaneous clients

Design Decisions

• TCP only, WebSocket only, or both?
• mDNS/DNS-SD advertisement for auto-discovery?
• Authentication/TLS for non-localhost connections?

Related

• arch: multi-platform architecture — shared core, multiple runtimes #17 — multi-platform architecture
• feat: Kismet companion process for surveillance detection #12 — Kismet companion (may connect via this listener)