Grant release 6's compositions, wants and backups ahead of its switch (issue 1945)

The switch to release 6 is today's controller's last plan, and the bus runs the user list it composes until the new
controller writes one: so the new controller's writes of each node's compositions and their digests, each new
node-engine's reads of its own compositions and wants and its backup holder's verbs and word, and the assigner's read
of the digests must be granted by this controller already (ADR 0539 clauses 2, 6, 9 and 16; the review's A1).

internal/broker/controller_buckets.go

@@ -196,14 +196,15 @@ func IsControllerBucket(bucket string) bool {
 		bucket == MergeWindowBucket || bucket == RolloutSilentBucket ||
 		bucket == DeliveryMergesBucket || bucket == BuildsBucket || bucket == RolloutsBucket ||
 		bucket == delivery.HaveBucket || bucket == StatesAnnouncedBucket ||
-		bucket == GrantsBucket
+		bucket == GrantsBucket || bucket == ControllerComposedBucket
 }
 
 // ControllerBuckets are the controller's own buckets, in the order they are asserted.
 func ControllerBuckets() []string {
 	return []string{LeaseBucket, CallsBucket, HandActsBucket, ConditionsBucket, ConditionHistoryBucket, AskedBucket,
 		AssignmentsBucket, DeclaredConfigBucket, PredefinedBucket, DeclaredPackagesBucket, MergeWindowBucket, RolloutSilentBucket,
-		DeliveryMergesBucket, BuildsBucket, RolloutsBucket, delivery.HaveBucket, StatesAnnouncedBucket, GrantsBucket}
+		DeliveryMergesBucket, BuildsBucket, RolloutsBucket, delivery.HaveBucket, StatesAnnouncedBucket, GrantsBucket,
+		ControllerComposedBucket}
 }
 
 // ControllerBucketsAsserter is what raising the controller's buckets needs of a connection.
@@ -331,6 +332,10 @@ func (j *JetStream) EnsureControllerBuckets() error {
 	}); err != nil {
 		return fmt.Errorf("asserting bucket %s: %w", RolloutSilentBucket, err)
 	}
+	// **The compositions' digests** (ADR 0539 clauses 2 and 6), asserted ahead of release 6: what its assigner reads.
+	if err := EnsureComposedDigests(ctx, js); err != nil {
+		return err
+	}
 	// **The states each module announced** (novox/hq ADR 0495 clauses 2 and 3, issue 1790): one key a module, its last word.
 	if _, err := js.CreateOrUpdateKeyValue(ctx, jetstream.KeyValueConfig{
 		Bucket: StatesAnnouncedBucket,