Keep what a consumer gives up on until a person delivers it again or drops it

Design 25 promised a dead-letter stream that did not exist: a message a
consumer gave up on stayed only in its source, which drops it after a week,
and its condition cleared when the advisories stopped (hq issue 330, ADR 0264).

cmd/mesh-controller/watchdogs.go

@@ -75,6 +75,9 @@ type signalFacts struct {
 
 	advisories    []link.Advisory
 	lostConsumers map[string]bool
+	// deadLetters are how many messages DEAD_LETTERS holds per consumer, by `<stream>.<consumer>`
+	// (novox/hq issue 330): each consumer's max-deliveries condition is open while it holds any.
+	deadLetters   map[string]int
 	advisoriesErr error
 
 	selfCheck selfCheckFacts
@@ -332,6 +335,9 @@ func (w *watchdogs) gather(ctx context.Context) *signalFacts {
 	}
 	f.advisories = link.Advisories.Since(now.Add(-advisoryQuiet))
 	f.lostConsumers, f.advisoriesErr = w.lostConsumers(ctx, f.advisories)
+	if f.advisoriesErr == nil && w.js != nil {
+		f.deadLetters, f.advisoriesErr = link.HeldDeadLetters(w.js.Context())
+	}
 	f.handActs, f.handActsErr = w.gatherHandActs(ctx, now)
 	f.facts.taken, _, f.facts.began, f.facts.err = exportedFacts.last()
 	return f
@@ -654,6 +660,8 @@ func watchTheMesh(ctx context.Context, open *stores, server *link.Server, bus li
 		return nil
 	}
 	conditionsFrom = keeper
+	// What a consumer gave up on is read and acted on by the verb in this process (novox/hq issue 330).
+	deadLettersOn.Store(&busHandles{conn: server.JetStream().Conn(), js: server.JetStream().Context()})
 	logf := func(format string, args ...any) { fmt.Printf(format+"\n", args...) }
 	stopHearing, err := server.HearAdvisories(logf)
 	if err != nil {