Gate a release plan's first machine and roll a failed build back there (hq ADR 0236)

A build that reported applied was sent everywhere; one that then did nothing, served
no tools or broke its machine's word reached every machine. Now the first machine is
judged by the component's health (the core's definitions, as doctor probes H-*, or a
module's own) three times over two minutes within ten; a failing gate puts the previous
build back there once, marks the build, and says it as a condition and an event.
Upgrades roll out by default; the bus is a planned step; a module deleted at its
source is not built (the public-acme plan failure).

cmd/mesh-controller/rollout.go

@@ -388,14 +388,7 @@ func rolloutMint(ctx context.Context, again bool) error {
 }
 
 // providesBus is whether a manifest provides the mesh's bus.
-func providesBus(m catalogue.Manifest) bool {
-	for _, o := range m.Provides {
-		if o.Name == "mesh-bus" {
-			return true
-		}
-	}
-	return false
-}
+func providesBus(m catalogue.Manifest) bool { return catalogue.ProvidesBus(m) }
 
 // rolloutHand mints a machine its credential for the new bus afresh and prints its membership
 // once, for an operator to carry by hand — the rescue for a machine that cannot be reached over