Function
A stable set of completed lesson ids; empty before hydration
The shape a count needs. useLessonCompletion answers for one lesson, which is right for an indicator and wrong for a module meter that must ask about seventeen at once — and a hook call per lesson would break the rules of hooks the moment a module is reordered.
Browser-side only — do NOT call from a Server Component.
Every lesson the signed-in learner has completed, as one snapshot.