Raise the bus module's watch when it falls silent or says it cannot see (hq issue 270, review 274)

A dead watch of slow consumers reads as none. The watch now beats after every
look; S9 reads the newest heartbeat and raises bus-watch-dead when it is older
than two minutes or says the watch is not healthy, on the bus's machine. A
stream with no heartbeat at all is a bus module older than its watch.

internal/link/busfaults.go

@@ -32,6 +32,9 @@ const BusModule = "nats"
 const (
 	BusSlowConsumerEvent     = "bus.slow-consumer"
 	BusConnectionClosedEvent = "bus.connection-closed"
+	// BusWatchEvent is the bus module's watch saying it is there after every look, and whether it is healthy
+	// (novox/hq ADR 0357, review 274): a watch that stops saying it is a watch nobody would otherwise hear.
+	BusWatchEvent = "bus.watch"
 )
 
 // AdvisoryConnectionClosed is a connection the bus closed for breaking its rules, as conditions name it.
@@ -233,3 +236,33 @@ func MergeAdvisories(a, b []Advisory) []Advisory {
 	}
 	return out
 }
+
+// BusWatchBeat is the bus module's watch's newest heartbeat.
+type BusWatchBeat struct {
+	At      time.Time `json:"at"`
+	Healthy bool      `json:"healthy"`
+	Why     string    `json:"why,omitempty"`
+	// Machine is the machine the runtime stamped on it (`x-node`).
+	Machine string `json:"-"`
+}
+
+// BusWatchLast is the newest heartbeat of the bus module's watch, read directly from the events stream; false
+// when the stream holds none — a bus module older than its watch, which says nothing either way.
+func (s *Server) BusWatchLast(ctx context.Context) (BusWatchBeat, bool, error) {
+	if s.js == nil {
+		return BusWatchBeat{}, false, errors.New("this control plane is not on the bus, so it cannot read the bus module's watch")
+	}
+	msg, err := s.js.Context().GetLastMsg(broker.EventsStream, EventSubject(BusModule, BusWatchEvent), nats.Context(ctx))
+	if errors.Is(err, nats.ErrMsgNotFound) {
+		return BusWatchBeat{}, false, nil
+	}
+	if err != nil {
+		return BusWatchBeat{}, false, fmt.Errorf("reading the bus module's watch's heartbeat: %w", err)
+	}
+	var b BusWatchBeat
+	if err := json.Unmarshal(msg.Data, &b); err != nil || b.At.IsZero() {
+		b = BusWatchBeat{At: msg.Time, Why: "its heartbeat could not be read"}
+	}
+	b.Machine = msg.Header.Get("x-node")
+	return b, true, nil
+}