feat(spec): add whenRoute action gating helper

whenRoute(route, allowedRoutes, body) wraps an actions() generator
that returns [] unless route.current matches one of the allowed
values. Accepts a single route or an array.

pkg/spec/src/index.ts

@@ -38,4 +38,5 @@ export {
   taps,
   waitOnce,
   weighted,
+  whenRoute,
 } from "./actions.ts";