Give each asking of a merge ask its own id, and add the have phase removed (issues 1370, 1369)

The router drops an ask under an id it keeps as a redelivery, so a gate that writes the same group's ask again at unchanged heads was never heard. Ask gains Asking; AskingID gives the first asking the old id and later ones their own. PhaseRemoved lets the node-engine say a module left its node.

go/delivery/delivery.go

@@ -80,6 +80,10 @@ const (
 	PhaseChecking  Phase = "checking"
 	PhaseRunning   Phase = "running"
 	PhaseFailed    Phase = "failed"
+	// PhaseRemoved is written by the node-engine, once, for a module its applied declaration no longer holds and
+	// for which it had written a have key; the key is never deleted (issue 1369, ADR 0408 §4), so a reader can tell
+	// that the module stopped.
+	PhaseRemoved Phase = "removed"
 )
 
 // Health is what the module's own check says.
@@ -338,9 +342,9 @@ func decode(data []byte, into any) error {
 
 func (h Have) check() error {
 	switch h.Phase {
-	case PhaseBackingUp, PhaseApplying, PhaseChecking, PhaseRunning, PhaseFailed:
+	case PhaseBackingUp, PhaseApplying, PhaseChecking, PhaseRunning, PhaseFailed, PhaseRemoved:
 	default:
-		return fmt.Errorf("delivery: phase %q is not backing-up, applying, checking, running or failed", h.Phase)
+		return fmt.Errorf("delivery: phase %q is not backing-up, applying, checking, running, failed or removed", h.Phase)
 	}
 	switch h.Health {
 	case HealthOK, HealthFailed, HealthWaits, HealthUnknown: