fix(client): keep the run's exit signal readable after supervisor Stop

Stop cleared the done channel, so a waiter that raced a teardown got nil
from Done() and selected on a nil channel: an Up racing a Down hung in
waitForUp for its full 50s timeout where it previously failed fast with
"client gave up to connect". The same clearing made Alive() report a
timed-out run dead while its goroutine was still tearing down, letting
Up start an overlapping second run.

Keep the channel in place instead: the exiting run closes it, so late
waiters observe the real exit and Alive() stays true until the run is
actually gone — the lifecycle clientGiveUpChan used to have. Stop still
drops the current client and invalidates the generation.

The stale-running-chan test begins a fresh run before waiting, as Up
does, so the kept exit signal does not race the stale clientRunningChan.

client/server/server_connect_test.go

@@ -190,6 +190,11 @@ func TestDownThenUp_StaleRunningChan(t *testing.T) {
 	assert.False(t, s.clientRunning, "clientRunning should be cleared by cleanupConnection (intent = down)")
 	s.mutex.Unlock()
 
+	// A fresh Up begins a new run before waiting; without it the previous
+	// run's kept exit signal would race the stale clientRunningChan below.
+	_, nextDone := s.runs.Begin()
+	defer close(nextDone)
+
 	// waitForUp() returns immediately due to stale closed clientRunningChan
 	waitCtx, ctxCancel := context.WithTimeout(context.Background(), 2*time.Second)
 	defer ctxCancel()