Hold the build seat's checks while a delivery builds, and let one check that waited 30 minutes through

ADR 0483: the controller keeps a key per plan in its build phase; holders read it and
take no new check while one is there, but for the oldest check past 30 minutes, one
at a time by a let-through key. The queue says why checks wait.

internal/broker/controller_buckets.go

@@ -63,6 +63,26 @@ var (
 	// repository: the controller reads the ask's warrant from the router and accepts only the operator's Approve of
 	// those heads. Written by the controller alone, from the announcement, when it holds the lease.
 	DeliveryMergesBucket = BucketName(ControllerSeat, "delivery-merges")
+	// PlansBuildingBucket holds one key per delivery plan in its build phase (novox/hq ADR 0483 clause 2, issue 1687): the key
+	// is `<plan id>` (`plan-<digits>`), its value the plan, the time its build phase began and the time of the last refresh.
+	// The controller (lease holder) puts it when the plan's first build is asked, puts it again about every thirty seconds
+	// while the plan builds, and deletes it when the phase ends (every build of the last tier built, or the plan fails, stops
+	// or waits on the operator); the bucket's age, PlansBuildingKeptFor, is how long a key outlives a controller that
+	// stopped. **Read by the build seat's holders and the checker, written by the controller alone**: a holder takes no new
+	// check while any key is there, but for one that waited past ADR 0483's bound. A reader lists the keys and reads none of
+	// their values to decide that (`link.BuildingPlans`).
+	PlansBuildingBucket = BucketName(ControllerSeat, PlansBuilding)
+)
+
+// PlansBuilding is the local name of the bucket of the plans building, as a reader names it in `reads`
+// (`mesh-controller.plans-building`).
+const PlansBuilding = "plans-building"
+
+// PlansBuildingKeptFor is the age of a plan-building key: the controller puts it again at PlansBuildingRefresh, so a
+// controller that stopped leaves checks held at most this long (ADR 0483 clause 2).
+const (
+	PlansBuildingKeptFor = 2 * time.Minute
+	PlansBuildingRefresh = 30 * time.Second
 )
 
 // DeliveryMergesKeptFor is how long a merge mesh-delivery announced is kept: a week, as the events are.
@@ -204,14 +224,14 @@ func IsControllerBucket(bucket string) bool {
 		bucket == AssignmentsBucket || bucket == DeclaredConfigBucket || bucket == PredefinedBucket ||
 		bucket == MergeWindowBucket || bucket == RolloutSilentBucket || bucket == CheckRunningBucket ||
 		bucket == DeliveryMergesBucket || bucket == BuildsBucket || bucket == RolloutsBucket ||
-		bucket == delivery.HaveBucket
+		bucket == PlansBuildingBucket || bucket == delivery.HaveBucket
 }
 
 // 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, MergeWindowBucket, RolloutSilentBucket, CheckRunningBucket,
-		DeliveryMergesBucket, BuildsBucket, RolloutsBucket, delivery.HaveBucket}
+		DeliveryMergesBucket, PlansBuildingBucket, BuildsBucket, RolloutsBucket, delivery.HaveBucket}
 }
 
 // ControllerBucketsAsserter is what raising the controller's buckets needs of a connection.
@@ -354,6 +374,22 @@ func (j *JetStream) EnsureControllerBuckets() error {
 	}); err != nil {
 		return fmt.Errorf("asserting bucket %s: %w", CheckRunningBucket, err)
 	}
+	// **The plans building: keys that age** (ADR 0483 clause 2, issue 1687). One message a key, each refresh a new one, so the
+	// age of the bucket is how long a key outlives a controller that stopped refreshing it.
+	if _, err := js.CreateOrUpdateKeyValue(ctx, jetstream.KeyValueConfig{
+		Bucket: PlansBuildingBucket,
+		Description: "the delivery plans in their build phase (novox/hq ADR 0483): `<plan id>` holds the plan and when its " +
+			"build phase began; put when the plan's first build is asked and about every thirty seconds, deleted when the " +
+			"phase ends, gone within two minutes of a controller that stopped; read by the build seat's holders, which " +
+			"take no new check while a key is there; written by the controller alone",
+		History:      1,
+		TTL:          PlansBuildingKeptFor,
+		MaxValueSize: 4 << 10,
+		MaxBytes:     8 << 20,
+		Storage:      jetstream.FileStorage,
+	}); err != nil {
+		return fmt.Errorf("asserting bucket %s: %w", PlansBuildingBucket, err)
+	}
 	// **The merges mesh-delivery announced: keys that age** (ADR 0420 §7, issue 1025). One message a key; a week.
 	if _, err := js.CreateOrUpdateKeyValue(ctx, jetstream.KeyValueConfig{
 		Bucket: DeliveryMergesBucket,