Hold a walk's first-machine gate while its busy container finishes its build

A merge into mesh-controller replaced every build agent and threw away the
checks in flight (issue 594). The manifest's container busy is checked and
sent only to an engine that reads it; a report's busy waits are kept, the
gate starts at the report that says the container replaced, and the wait is
a phase of the delivery time. mesh-builder busy drains the agent and answers
while a build is in hand (novox/hq ADR 0335).

cmd/mesh-controller/plan.go

@@ -975,9 +975,14 @@ func renderingFor(ctx context.Context, open *stores, node string,
 	if err != nil {
 		return catalogue.Rendering{}, inventory.Node{}, err
 	}
+	readsBusy, err := engineReadsBusy(ctx, inv, node)
+	if err != nil {
+		return catalogue.Rendering{}, inventory.Node{}, err
+	}
 	return catalogue.Rendering{
 		ReadsHealth:   readsHealth,
 		JudgesRoot:    judgesRoot,
+		ReadsBusy:     readsBusy,
 		BusMembership: memberships[node],
 		Settings:      settings, Generators: gens, Grants: grants, Needed: needed, Foreseen: foreseen, Ports: ports,
 		Certificate: certificate, Authority: authority, Mesh: private, Names: names,
@@ -1010,6 +1015,16 @@ func engineJudgesRoot(ctx context.Context, inv *inventory.Inventory, node string
 	return had && stated.Contract >= link.RootContract, nil
 }
 
+// engineReadsBusy says whether a machine's node-engine reads a container's `busy` (novox/hq ADR 0335), by its
+// own newest statement, for the same reason as engineReadsHealth: an older engine parses strictly.
+func engineReadsBusy(ctx context.Context, inv *inventory.Inventory, node string) (bool, error) {
+	stated, had, err := inv.HealthOf(ctx, node)
+	if err != nil {
+		return false, err
+	}
+	return had && stated.Contract >= link.BusyContract, nil
+}
+
 // zonesInTheMesh is every zone a module in the mesh declares, where the mesh placed it (novox/hq ADR
 // 0199): the zone settled from that node's settings, the node's private address, the port the
 // answering listen is published on there.