Hold a group's batch open for its last member and keep both kinds of member in one batch and one plan (issue 977, part C, ADR 0418)

internal/inventory/plans.go

@@ -139,6 +139,12 @@ type PlanBatch struct {
 	// Waits says why a batch made for one merge waits for the window: its delivery group has a member still to
 	// merge, or mesh-delivery could not say whether it has (novox/hq ADR 0306 decision 2).
 	Waits string `json:"waits,omitempty"`
+	// Holds are the groups whose last member is still to merge, by head branch name, each with why in words
+	// (novox/hq ADR 0418 clause 3): the batch does not close by the merge window while one is held, however long the
+	// member takes. Cleared when every member is merged or closed; the window then runs again from that moment.
+	Holds map[string]string `json:"holds,omitempty"`
+	// HeldAt is when the last hold was cleared: the maximum is counted from it, not from the batch's making.
+	HeldAt time.Time `json:"held_at,omitempty"`
 }
 
 // OwnPath says the record is a batch of merges on the controller's own path.