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.

cmd/mesh-controller/push.go

@@ -284,6 +284,8 @@ func serve(ctx context.Context) (err error) {
 	go mirroringRecords(ctx, inv, bus.Conn, bus)
 	// And the bucket of the files that travel by digest, judged against what each machine's latest declaration names (ADR 0486 §3).
 	go sweepingDeclarationFiles(ctx, inv, bus.Conn)
+	// And the key of each plan in its build phase, which holds the build seat's checks while a delivery builds (ADR 0483 clause 2).
+	go keepingPlansBuilding(ctx, inv, bus.Conn)
 	// Every call carries the lease's epoch, and its record is written only under the lease (novox/hq
 	// to-be 45 §6).
 	link.Calls.UnderLease(func() (uint64, error) { return theLease.epoch(ctx) })