Machine client

The client implements the bottom tier of the pattern and is the only component that knows the machine's native protocol. It is consumed as a library inside the worker, translating instructions down to the machine over TCP and passing machine-originated messages back up.

Isolating protocol knowledge in one library means a new machine type needs a new client, not a new platform. Reconnect, backoff, and heartbeat handling live here so that a dropped socket recovers without operator intervention.