Package-level declarations

Injected into each step so it can talk to its host without knowing what renders it: steps call the methods, the host observes events.

How a step ended. Only COMPLETED is persisted as a completed step.

A single step in the native subscription onboarding flow, contributed from the feature module that owns the step (e.g. the VPN step lives in the VPN module, the Duck.ai step in the Duck.ai module). Each feature module contributes one implementation as a multibinding; the host in subscriptions-impl collects them, orders them (by @PriorityKey), and turns them into linear-onboarding steps.