Tell the operator at once when sends wait for the bus's planned step, and say it before the merge (hq issue 336)

For 28 minutes on 2026-10-08 every send to the control-node was refused for a new bus build that only a
person's bus upgrade moves, and no condition said so: the refusal lived only in each walk's note, and S3
would have called it lateness after half an hour, in words that named neither the bus nor the verb.

- Row S17, bus.<module>.step-waiting: raised on the first watchdog tick after a walk's send is refused for
  the bus, for the operator, naming the machines, what waits, the bus build from and to, since when and
  mesh-controller.bus upgrade. It clears once the bus's machine runs the build the mesh holds.
- S3 leaves out a walk held only by the bus's step.
- A change that builds the bus says in its delivery plan and summary (which mesh/merge-gate carries) that
  merging it needs a person's bus upgrade, and that nothing else reaches its machine until then.
- TestReplay336 fails on the commit before and passes on this one.

cmd/mesh-controller/release_plan.go

@@ -533,6 +533,10 @@ func advanceHeld(ctx context.Context, open *stores) {
 				// the state is left as it was and the step is tried again on the next tick. Said and
 				// kept when it is new: the same refusal on every tick is one fact, not one per tick.
 				p.Note = "tier " + fmt.Sprint(p.Tier) + ": " + err.Error() + " — tried again"
+				// A refusal for the bus's planned step is a person's to end: S17 says it from its first moment.
+				if refusedForTheBus(err) {
+					busRefusedFirst.mark(p.ID, time.Now())
+				}
 				if planSnapshot(*p) != before {
 					fmt.Printf("%s: %v\n", p.ID, err)
 					if err := inv.SavePlan(ctx, p); err != nil {