Keep what discovery heard when a round hears nothing, and fetch only changed announcements (issue 554)

A round in which no PING reached the laptop's console within the 750 ms window
was taken as the mesh's answer that nothing is there, and replaced the index:
every seat and module call was refused for five seconds at a time. Discovery
now waits up to its patience for a first PING, a round that heard nothing is
not kept, a runtime not heard this round is resolved from what it announced
last (said with its age), and a round that misses runtimes is logged once.
A PING carries a digest of what the instance serves, so an unchanged
announcement is not pulled again every round (some 880 kB to the laptop).

node-tools/internal/bus/bus.go

@@ -775,6 +775,21 @@ type Arrival struct {
 // that said it is there, not for a fixed moment (2026-10-05). Answers beyond what the caller has
 // taken are kept up to the channel's room; an empty answer is no answer.
 func (c *Conn) Collect(subject string, body []byte) (<-chan Arrival, func(), error) {
+	out, ask, stop, err := c.Collector(subject)
+	if err != nil {
+		return nil, func() {}, err
+	}
+	if err := ask(subject, body); err != nil {
+		stop()
+		return nil, func() {}, err
+	}
+	return out, stop, nil
+}
+
+// Collector is one inbox for many requests: every answer to every request `ask` publishes arrives on
+// the one channel, until stopped — for discovery, which asks each service that said it is there for
+// what it serves, as each says so (novox/hq issue 554). `what` names the requests in a log line.
+func (c *Conn) Collector(what string) (<-chan Arrival, func(subject string, body []byte) error, func(), error) {
 	inbox := c.nc.NewRespInbox()
 	out := make(chan Arrival, 1024)
 	var mu sync.Mutex
@@ -792,11 +807,11 @@ func (c *Conn) Collect(subject string, body []byte) (<-chan Arrival, func(), err
 		select {
 		case out <- a:
 		default:
-			c.Logf("[mesh-tools] more answers to %s than are kept; one (%d bytes) dropped", subject, len(msg.Data))
+			c.Logf("[mesh-tools] more answers to %s than are kept; one (%d bytes) dropped", what, len(msg.Data))
 		}
 	})
 	if err != nil {
-		return nil, func() {}, err
+		return nil, nil, func() {}, err
 	}
 	stop := func() {
 		_ = sub.Unsubscribe()
@@ -804,11 +819,8 @@ func (c *Conn) Collect(subject string, body []byte) (<-chan Arrival, func(), err
 		stopped = true
 		mu.Unlock()
 	}
-	if err := c.nc.PublishRequest(subject, inbox, body); err != nil {
-		stop()
-		return nil, func() {}, err
-	}
-	return out, stop, nil
+	ask := func(subject string, body []byte) error { return c.nc.PublishRequest(subject, inbox, body) }
+	return out, ask, stop, nil
 }
 
 // MaxPayload is the largest message the bus carries, as the server told this connection.