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).

internal/broker/users.go

@@ -72,7 +72,13 @@ func Users(r Records) ([]Principal, error) {
 	out := []Principal{{Kind: KindController}}
 
 	for _, node := range sortedCopy(r.Nodes) {
-		out = append(out, Principal{Kind: KindNode, Node: node})
+		witness := false
+		for _, d := range r.Assigned[node] {
+			if d.Module == controllerModule {
+				witness = true
+			}
+		}
+		out = append(out, Principal{Kind: KindNode, Node: node, WitnessesController: witness})
 		// **Where the runtime is assigned, the machine gets one runtime principal in place of the
 		// runtime module's own** (novox/hq ADR 0175, to-be 38). It carries every module on the
 		// node: its serving grants are the union of theirs. Every other module keeps its own
@@ -171,3 +177,6 @@ func sortedNames(in map[string][]string) []string {
 	sort.Strings(out)
 	return out
 }
+
+// controllerModule is the controller's module: the machine assigned it witnesses its upgrades.
+const controllerModule = "mesh-controller"