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/signals.go

@@ -210,6 +210,20 @@ var signalsTable = []signalRow{
 		newest: func(f *signalFacts) time.Time {
 			return newestOf(f.waits, func(w waitFacts) time.Time { return w.since })
 		}},
+	{Row: "S17", Signal: "a send held for the bus's planned step is told to a person", Emitter: "controller's plan",
+		Trigger: "each send refused because it would replace the bus outside its step (novox/hq issue 336)",
+		Bound: "none: raised at the first refusal, for the operator, naming what waits, the bus build from and to, " +
+			"since when and `bus upgrade`; cleared once the bus's machine runs the build the mesh holds",
+		Kind: kindBusStepWaiting, Severity: conditions.Warning, Phase: 3,
+		needs: func(f *signalFacts) error {
+			if f.plansErr != nil {
+				return f.plansErr
+			}
+			return f.busErr
+		}, watch: watchBusWaits,
+		newest: func(f *signalFacts) time.Time {
+			return newestOf(f.bus.waits, func(w busWaitFacts) time.Time { return w.since })
+		}},
 }
 
 // watchFacts is S14: the snapshot a merge check is fed is older than its bound, or none was kept since
@@ -315,7 +329,9 @@ func watchReports(f *signalFacts) []conditions.Observation {
 func watchPlans(f *signalFacts) []conditions.Observation {
 	var out []conditions.Observation
 	for _, p := range f.plans {
-		if p.paused {
+		// Under a paused build seat, or held only by the bus's planned step (S17, novox/hq issue 336): a wait
+		// for a person, said as itself, not a walk running late.
+		if p.paused || p.bus {
 			continue
 		}
 		in := f.now.Sub(p.entered)