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/push.go

@@ -165,6 +165,10 @@ func serve(ctx context.Context) (err error) {
 	// Open plans move on a timer as well as on outcomes (novox/hq ADR 0162): a tier waiting for
 	// machines to report moves when they have, and a plan left by a replaced controller resumes.
 	go planTicker(ctx, open)
+	// And the pending assignments settled on a tick of their own (novox/hq ADR 0261): made once their module
+	// is registered, ended with why when its build will not register it, raised and cleared as conditions.
+	// Never by a read.
+	go settlingPending(ctx, open)
 	// The durations the core's bounds are set from are kept a month (novox/hq to-be 45 Phase 0).
 	go forgettingOldDurations(ctx, inv)
 	// And what the catalogue decided a build meant. The builder's own result is already handled