Advise a recheck for an unanswered check on a ready or rejected delivery (issue 451)

That check answered once, so the build queue is the wrong place to look.
mesh-delivery names the delivery's state as data (held); the controller
reads it and never the words. A line without it keeps the advice of 438.

cmd/mesh-controller/delivery_conditions.go

@@ -47,10 +47,14 @@ type stalledLine struct {
 	// issue 443): a time inside a finished sentence cannot be said again in another zone. A mesh-delivery from before
 	// says none, and Waiting is said as it reads.
 	Checks []waitingCheck `json:"checks,omitempty"`
-	For    string         `json:"for"`
-	Bound  string         `json:"bound"`
-	H2     string         `json:"h2"`
-	Says   string         `json:"says"`
+	// Held is the state of the delivery of a line of the state `unanswered`, ready or rejected, when one is held
+	// (novox/hq issues 441, 451): its check answered once, so a recheck is the step. Empty for a head with no
+	// delivery, and from a mesh-delivery that does not say it yet; the words are never read for it.
+	Held  string `json:"held,omitempty"`
+	For   string `json:"for"`
+	Bound string `json:"bound"`
+	H2    string `json:"h2"`
+	Says  string `json:"says"`
 }
 
 // waitingCheck is one merge check a stalled line waits on, as mesh-delivery gives it: its context, its state —