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/compose_sdk.go

@@ -366,7 +366,7 @@ func sayFound(ctx context.Context, d compose.Declared) {
 
 // sendableOf is a composed declaration as the controller's sends know it.
 func sendableOf(d compose.Declared) sendable {
-	out := sendable{Resources: d.Resources, Sequence: d.Sequence, Epoch: d.Epoch, Generation: d.Generation,
+	out := sendable{Resources: compactKept(d.Resources), Sequence: d.Sequence, Epoch: d.Epoch, Generation: d.Generation,
 		ReadsModules: d.ReadsModules, moduleOf: d.ModuleOf, buildIDs: d.BuildIDs, modules: d.Modules,
 		localNetwork: d.LocalNetwork, Received: d.Received, Mesh: d.Mesh, BusUsers: d.BusUsers,
 		LeftOut: d.LeftOut, leftOutWhy: d.LeftOutWhy, withheld: d.Withheld, unbound: d.Unbound, foreseen: d.Foreseen,