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_test.go

@@ -142,6 +142,19 @@ var suppressions = map[string]suppression{
 				since: f.now.Add(-31 * time.Minute)}}
 		},
 	},
+	// A send refused because it would replace the bus outside its planned step: said at its first refusal,
+	// whatever the bound (novox/hq issue 336). Inside: a new bus build waits, and no send was refused for it.
+	"S17": {
+		inside: func(f *signalFacts) {
+			f.bus = busFacts{module: "nats", to: "32307bd1bbbb", from: map[string]string{"anchor": "88135ad0aaaa"},
+				machines: []string{"anchor"}}
+		},
+		past: func(f *signalFacts) {
+			f.bus = busFacts{module: "nats", to: "32307bd1bbbb", from: map[string]string{"anchor": "88135ad0aaaa"},
+				machines: []string{"anchor"}, waits: []busWaitFacts{{plan: "plan-1", repository: "novox/app",
+					commit: "c0ffee001122", modules: []string{"app"}, since: f.now.Add(-time.Second)}}}
+		},
+	},
 	// Twice by hand within a fortnight is a healer wanted; once, or the first of two a day too old, is not.
 	"S15": {
 		inside: func(f *signalFacts) {