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/plain_words.go

@@ -523,6 +523,13 @@ var plainWordings = map[string]func(conditions.Observation) words{
 			Explanation: "The mesh's message system is being upgraded; some things pause until it is done.",
 			Resolved:    "The bus upgrade is done"}
 	}),
+	kindBusStepWaiting: worded(func(o conditions.Observation) words {
+		return words{Headline: "Sends wait for a bus upgrade",
+			Needs: "start the bus upgrade " + FromMeshMCPServer,
+			Explanation: "A new version of the mesh's message system is built, and only a person installs it. Until " +
+				"then nothing else is sent to the machine that runs it.",
+			Resolved: "Resolved: the bus upgrade started, and sends go on"}
+	}),
 	kindBusUpgradeFailed: worded(func(o conditions.Observation) words {
 		return words{Headline: "The bus upgrade failed",
 			Needs:       "decide whether to put the bus back to the version before; the details say how.",