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/gate.go

@@ -1443,6 +1443,8 @@ func probeGates(ctx context.Context, d *doctor) ([]conditions.Observation, error
 	}
 	// A release held after a failed one, while builds still wait for a gate (ADR 0236).
 	out = append(out, backlogObservation()...)
+	// And a rest that went silent (novox/hq ADR 0353 §4).
+	out = append(out, silentObservation(ctx, d.open.inventory)...)
 	return sortedFound(dedupeObservations(out)), nil
 }