Land feature 57's node-engine work on its release branch

The node-engine renders the feature's templates, collections and moved
providers' files, on the SDK the feature's modules build on (feature 57,
config and provisioning).

internal/system/system.go

@@ -304,6 +304,27 @@ func For(name string) (System, error) {
 		name, strings.Join(names, ", "))
 }
 
+// userManagers is a service manager that runs a manager per account (novox/hq ADR 0177).
+type userManagers interface {
+	UserManagerRunning(ctx context.Context, run Runner, account string) (running, exists bool, err error)
+}
+
+// ManagerRunning is whether account's own manager runs on a machine built for name, asked of the
+// machine's manager through run — never of the account's, which a question would start (see
+// arch.UserManagerRunning). A look asks it before it reads anything in an account's manager (novox/hq
+// issue 764). A system with no manager per account is an error: nothing can be read there.
+func ManagerRunning(ctx context.Context, name string, run Runner, account string) (running, exists bool, err error) {
+	sys, err := For(name)
+	if err != nil {
+		return false, false, err
+	}
+	um, ok := sys.(userManagers)
+	if !ok {
+		return false, false, fmt.Errorf("this machine's service manager (%s) has no manager per account", sys.Name())
+	}
+	return um.UserManagerRunning(ctx, run, account)
+}
+
 // All is every system the host can be built for.
 func All() []System {
 	return []System{arch{}, alpine{}, android{}}