[client] fix Start/Stop race by making the run loop own engine shutdown

ConnectClient.Stop stopped the engine directly while the run loop's
backoff cycle could still be starting an engine, so Engine.close raced
Engine.Start (e.g. firewall setup reading wgInterface while close nils
it). embed.Client.Start's rollback only avoided a deadlock by cancelling
before Stop; the race itself remained and was caught by -race.

Make the run loop the sole owner of engine shutdown: derive the run
context in NewConnectClient, and have Stop cancel it and wait for the
loop to exit (skipping the wait when the loop never ran) instead of
calling engine.Stop. The loop now always stops the engine on its way
out, dropping the unsynchronised wgInterface check it used to guard that
call. Self-calls from within the loop use runCancel to avoid waiting on
themselves.

embed keeps a defensive pre-Stop cancel(); the daemon's cleanupConnection
gets a TODO to adopt Stop() rather than stopping the engine in parallel.

client/server/server.go

@@ -988,6 +988,10 @@ func (s *Server) cleanupConnection() error {
 		return nil
 	}
 
+	// TODO: consider calling s.connectClient.Stop() instead of engine.Stop().
+	// actCancel() lets the run loop stop the engine too, so both stop it
+	// concurrently; ConnectClient.Stop cancels and waits for the run loop,
+	// making the run loop the sole owner of engine shutdown.
 	if engine != nil {
 		if err := engine.Stop(); err != nil {
 			return err