Accept waits for an SDK HELLO then hands back a Conn. Conn.Snapshot
sends a PAUSE, blocks on the matching STATE (id-correlated), and
leaves the SDK paused until Release sends RESUME. Conn.Close sends
GOODBYE best-effort.
v0.1 supports one client at a time; transport is left to the caller
so tests can use TCP loopback while production wires via adb reverse
to localabstract:uatu-agent.