Raise every bus user's slow consumer and closed connection as S9, from the bus module's events (hq issue 270, ADR 0357)

The controller heard a slow consumer only on its own connection: the server
says another user's only in its record of closed connections. The bus module
now says that record as events; the watchdogs read the last hour back on every
tick and raise one condition per user and kind, naming the module and machine.
The module check lets the bus module emit, since its tool runner publishes.

cmd/mesh-controller/plain_words.go

@@ -620,6 +620,12 @@ var plainWordings = map[string]func(conditions.Observation) words{
 			Explanation: "The bus reports a listener too slow to keep up, so messages to it are late.",
 			Resolved:    "The listener keeps up again"}
 	}),
+	"connection-closed": worded(func(o conditions.Observation) words {
+		return words{Headline: "The bus cut a client off",
+			Explanation: "The bus closed a client's connection because it broke the bus's rules, so what it was " +
+				"sending was lost.",
+			Resolved: "Resolved: the client keeps to the bus's rules"}
+	}),
 	kindBusReconnects: worded(func(o conditions.Observation) words {
 		return words{Headline: "A client keeps losing the bus",
 			Explanation: "One of the mesh's clients lost its connection to the bus again and again in the last hour. " +