- BackStackApp.linkStream: feed a plugin's runtime Stream<Uri> (custom scheme,
  Firebase Dynamic Links, warm app_links) through the same onLink, with the same
  fallback safety; back_stack owns the subscription lifecycle, stays zero-dep
- Add handleLink(uri) to NavStackRouterDelegate and MultiNavStackRouterDelegate,
  the imperative sibling of setNewRoutePath (which now delegates to it)