Make pending assignments safe to race, settle them on a tick, and say only what was checked

Review of #150: a withdrawal could land between the look and the act, a
failed ask read as a build in flight, a request kept the wrong asker, a
build being registered read as not built, build "true" could ask a build
nothing waited on, and a status read changed state. Claim a row under the
machine's hold before making it, keep a request only once asked, settle on
the controller's own tick, raise an assignment not made as a condition
until it is answered, and tie each row to its machine.

cmd/mesh-controller/plain_words.go

@@ -153,6 +153,19 @@ var plainWordings = map[string]func(conditions.Observation) words{
 				"asleep is normal.", m),
 			Resolved: m + " can be reached again"}
 	}),
+	// A pending assignment that was not made (novox/hq issue 325, ADR 0261).
+	kindPendingEnded: worded(func(o conditions.Observation) words {
+		m := machineOr(o, "a machine")
+		module := idPart(o, 1)
+		if module == "" {
+			module = "a module"
+		}
+		return words{Headline: module + " was not put on " + m,
+			Needs: "decide whether to build it again or take the assignment back; the details say why.",
+			Explanation: fmt.Sprintf("An assignment of %s to %s waited for its build, and the build did not "+
+				"register it, so it was not made.", module, m),
+			Resolved: "Resolved: " + module + " on " + m + " is settled"}
+	}),
 	kindBindingKept: worded(func(o conditions.Observation) words {
 		m := machineOr(o, "a machine")
 		return words{Headline: "A module's data source is held on " + m,