acquire
Synchronous because PaywallView.present is non-suspend.
Fast path: if state already holds the canonical view, return it without touching the actor queue. Slow path (cold start, or after removeAll evicted the tag): block on the actor's immediate so exactly one factory invocation happens across concurrent callers. The View is constructed on the actor thread; it is only attached to a hierarchy later, on the main thread, by PaywallView.