watcher: a removed nameserver is an NS Change, not an NS Failure (closes #262)

A nameserver that left a domain's delegation was reported twice: as the
domain's NS Change, and by the check of the domain's own records as an
error-level NS Failure saying it disappeared. That check now sends
nothing for a domain. For a hostname, the nameservers that leave its
zone are sent as one NS Change at warning level, with a Hostname: line
and a Removed: line, as decided on the issue; nothing is sent for one
that joins. The README says what each case gets.

A nameserver that was failing before it left is now listed too; the
NS Failure skipped it.
A domain with no delegation of its own gets nothing when a nameserver
leaves the zone it is in.

Model: opus-5-5

internal/watcher/watcher.go

@@ -772,28 +772,45 @@ func (w *Watcher) detectRecordChanges(
 	}
 }
 
+// detectNSDisappearances sends one NS Change for a hostname, listing
+// the nameservers of the previous check that are no longer those of its
+// zone. Nothing is sent for a configured apex domain, whose removed
+// nameservers detectNSChanges notifies.
 func (w *Watcher) detectNSDisappearances(
 	ctx context.Context,
 	hostname string,
 	prev, current *state.HostnameState,
 ) {
-	for ns, prevNS := range prev.RecordsByNameserver {
-		if _, ok := current.RecordsByNameserver[ns]; ok || prevNS.Status != statusOK {
-			continue
-		}
-
-		msg := fmt.Sprintf(
-			"%s\nNameserver: %s disappeared",
-			w.nameLine(hostname), ns,
-		)
-
-		w.notify.SendNotification(
-			ctx,
-			"NS Failure: "+hostname,
-			msg,
-			"error",
-		)
+	if w.isDomain(hostname) {
+		return
 	}
+
+	var removed []string
+
+	for ns := range prev.RecordsByNameserver {
+		if _, ok := current.RecordsByNameserver[ns]; !ok {
+			removed = append(removed, ns)
+		}
+	}
+
+	if len(removed) == 0 {
+		return
+	}
+
+	slices.Sort(removed)
+
+	msg := fmt.Sprintf(
+		"Hostname: %s\nRemoved: %s",
+		hostname,
+		strings.Join(removed, ", "),
+	)
+
+	w.notify.SendNotification(
+		ctx,
+		"NS Change: "+hostname,
+		msg,
+		"warning",
+	)
 }
 
 // detectNSFailures notifies when a nameserver that answered on the