fix: resolve silent message drop from session update race condition

Await the async sessionUpdate callback so errors propagate correctly instead of being silently swallowed. Buffer session update notifications that arrive before the session is registered in the map, and replay them once registration completes. Add .catch() to the message queue promise chain to prevent unhandled rejections.

Also verify at startup that netcat supports unix sockets (-U flag), failing fast with an actionable error message if the traditional (non-OpenBSD) variant is installed.