Let the kept compositions' pass end at the stop (issue 1945, ADR 0545, review 3)

followKept went on composing every kept key, and the config consumer every
touched node, after the controller was told to stop. Both now check the stop
before each key and each node; the writes of an unfinished batch are refused
at once, so the next controller takes them.
Research 072: milestone 2, the composer reacting to the change of an input.

cmd/mesh-controller/kept_compositions.go

@@ -137,6 +137,9 @@ func followKept(ctx context.Context, open *stores, node string, modules map[stri
 	var errs []error
 	builds := map[string][]inventory.Build{}
 	for _, k := range kept {
+		if ctx.Err() != nil {
+			return wrote, errors.Join(append(errs, ctx.Err())...) // stopped: the rest is the next controller's
+		}
 		if _, read := builds[k.module]; !read {
 			if builds[k.module], err = inv.Builds(ctx, k.module, 20); err != nil {
 				errs = append(errs, fmt.Errorf("%s's builds: %w", k.module, err))
@@ -311,6 +314,10 @@ func followConfigOnce(ctx context.Context, open *stores) error {
 			forgetResolved() // the next composition reads the values just written, not a read of the last 5 s
 			failed := false
 			for node, modules := range now {
+				if ctx.Err() != nil {
+					failed = true // stopped: the writes are refused below and delivered to the next controller
+					break
+				}
 				if err := composerFor(open).compose(ctx, []string{node}, false); err != nil {
 					fmt.Printf("config: %s is not composed after its config changed: %v\n", node, err)
 					failed = true
@@ -321,7 +328,9 @@ func followConfigOnce(ctx context.Context, open *stores) error {
 				}
 			}
 			for _, m := range msgs {
-				if failed {
+				if failed && ctx.Err() != nil {
+					_ = m.Nak() // stopped: delivered at once to the next controller
+				} else if failed {
 					_ = m.NakWithDelay(doneKeyRetry) // tried again: the bus delivers the writes again
 				} else {
 					acking, cancel := context.WithTimeout(context.WithoutCancel(ctx), 10*time.Second)