Let a walk wait for its delivery's word, and serve the delivery's owner (hq ADR 0239)

While the mesh-delivery seat has a holder on record, a merge that moves no
core module opens its walk and asks nothing until mesh-delivery or a person
says go; nothing of it is registered before its turn, so no other send
carries it. The controller keeps the planner, the gate, sending and the
walk, and gains the verbs the owner asks with: delivery-plan, -order,
-check (a group composed as one future state), deliver, delivery-stop,
delivery-walks; every walk kept is said as plan-moved.

internal/link/build.go

@@ -123,6 +123,21 @@ type CheckRequest struct {
 	// the controller, judged by itself; JudgeValidator for a change to the node-engine, judged by the
 	// running controller built with the change's validator in place of the one it vendors.
 	Judge string `json:"judge,omitempty"`
+	// Group names a delivery group whose members' heads are composed together with this one (novox/hq ADR
+	// 0239), and Members are those other heads, each cloned beside it and laid over the mesh in turn. The
+	// repository's own check is not run for a group: each member's pull request runs its own.
+	Group   string        `json:"group,omitempty"`
+	Members []GroupMember `json:"members,omitempty"`
+}
+
+// GroupMember is one other head of a delivery group's composed check.
+type GroupMember struct {
+	Owner      string   `json:"owner"`
+	Repo       string   `json:"repo"`
+	Number     int      `json:"number,omitempty"`
+	Repository string   `json:"repository"`
+	Ref        string   `json:"ref"`
+	Paths      []string `json:"paths,omitempty"`
 }
 
 // Who judges a merge check's gate.