Measure a declaration against the bus's largest message, raise a condition near it, and send the vault's export compact (issue 1689)

A declaration is one bus message and the control node's passed the limit by 2 KB. The size is checked at every send
against the connection's own limit, with a condition from 90% and a message naming size and limit when over; the
export, 138 KB compact of 165 KB indented, is sent compact.

cmd/mesh-controller/plain_words.go

@@ -195,6 +195,21 @@ var plainWordings = map[string]func(conditions.Observation) words{
 				"there yet is sealed by the controller until the vault holds it.", m, m),
 			Resolved: "Resolved: the vault gave " + m + " the core's credentials"}
 	}),
+	kindDeclarationNearLimit: worded(func(o conditions.Observation) words {
+		m := machineOr(o, "a machine")
+		if o.Severity == conditions.Urgent {
+			return words{Headline: m + " cannot be sent its instructions",
+				Needs: "make the instructions for " + m + " smaller, for example by taking a module off it.",
+				Explanation: fmt.Sprintf("The instructions for %s have grown past the largest message the bus takes, so "+
+					"nothing new reaches %s until they are smaller.", m, m),
+				Resolved: "Resolved: the instructions for " + m + " fit the bus again"}
+		}
+		return words{Headline: "The instructions for " + m + " are nearly too big",
+			Needs: "do not add more to " + m + " until this is dealt with; the details give the size.",
+			Explanation: fmt.Sprintf("The instructions for %s are close to the largest message the bus takes. "+
+				"One more module or secret may stop anything new from reaching %s.", m, m),
+			Resolved: "Resolved: the instructions for " + m + " have room again"}
+	}),
 	kindCoreDowngrade: worded(func(o conditions.Observation) words {
 		m := machineOr(o, "a machine")
 		component := idPart(o, 0)