Subscribe as soon as the runtime serves, never after a guessed wait, so no event sent at a start is missed (issue 774)

mesh-healers slept 2 s before its first subscribe to the keeper's conditions; its whole test delivered
condition-raised once the lease was read, often before that, and the event was dropped: "never: the would-act
written" in about two runs of three on main, failing the group check. Eight modules had the same shape.

Quick fix: sleep after a failed subscribe in mesh-healers alone.
Thorough fix (chosen, the kind is in eight modules): every loop that asks the runtime for events or a watch
waits for stdio.Served() and subscribes at once, sleeping only after a refusal (mesh-healers, mesh-config,
mesh-mail, mesh-issues, mesh-provisioner, mesh-watcher, mesh-delivery-gate); mesh-knowledge, whose pinned
mesh-sdk has no Served, subscribes first and sleeps after a refusal. The healers' whole test delivers the
event only once the bundle has subscribed it, as the runtime's consumer holds it until then.

modules/mesh-issues/cmd/mesh-issues/main.go

@@ -163,14 +163,15 @@ func hearOpened(t *Tracker) {
 	}
 	hearings = append(hearings, hearing{DeliveredEvent, t.OnDelivered})
 	for _, s := range hearings {
+		<-stdio.Served() // subscribed as soon as the runtime serves, never after a guessed wait: an event sent meanwhile is heard
 		for wait := 2 * time.Second; ; wait = min(wait*2, time.Minute) {
-			time.Sleep(wait)
 			err := stdio.Subscribe(s.pattern, s.handle)
 			if err == nil {
 				logf("[mesh-issues] the triage hears %s", s.pattern)
 				break
 			}
 			logf("[mesh-issues] the triage does not hear %s yet (%v); asking again in %s", s.pattern, err, wait)
+			time.Sleep(wait)
 		}
 	}
 }