Keep a walk's sends to what was kept, and say a silent rest (hq ADR 0353 review)

A release's own gates held its other sends back for ever; an adopted send
lost what the machine ran before; a kept send could grow a build no gate
passed; a take-over matched by commit alone; a rest that went silent ended
done. The node-engine and the resolver still go one machine at a time, since
a fault that cuts a machine off shows only as silence.

cmd/mesh-controller/plain_words.go

@@ -456,6 +456,9 @@ var plainWordings = map[string]func(conditions.Observation) words{
 	"release-held": worded(func(o conditions.Observation) words {
 		return releaseHeldWords(nil, o.Also)
 	}),
+	"release-silent": worded(func(o conditions.Observation) words {
+		return releaseSilentWords(o.Also)
+	}),
 	"facts-stale": worded(func(o conditions.Observation) words {
 		return words{Headline: "Merge checks use outdated facts",
 			Explanation: "The facts a merge check judges a change against are out of date; they are gathered again " +
@@ -841,6 +844,19 @@ func releaseHeldWords(modules, machines []string) words {
 		Resolved: "Resolved: the held updates are released"}
 }
 
+// releaseSilentWords are the plain words of machines that said nothing on an update they were sent (ADR 0353 §4).
+func releaseSilentWords(machines []string) words {
+	where := "Some machines"
+	if len(machines) > 0 {
+		where = conditions.Capital(namesWords(machines, 4))
+	}
+	return words{Headline: "Machines did not answer an update",
+		Needs: "check that they are awake and on the mesh; the next walk sends them what waits.",
+		Explanation: where + " were sent module updates and said nothing about them within fifteen minutes, so " +
+			"they were left as they were. What they run is not known to be the update.",
+		Resolved: "Resolved: the next walk reached them"}
+}
+
 // reloginNeeds is what an account waiting for its groups needs (ADR 0252).
 func reloginNeeds(node string) string {
 	return fmt.Sprintf("log out of %s completely and log in again, or restart it.", node)