A single lesson (legacy: LanguageLesson).

A marketing landing page for a course (legacy: LanguageLandingPage).

Every Lesson of the course's current version, in course order — the same list the course landing read returns.

The player's navigation tab is this list joined to progress.lessons by slug: names and order from here, checks and locks from there. Both screens render from the same two arrays, which is what keeps the Course page's list and the player's from disagreeing.

A learner's position in one Course, computed against its current Version.

Availability travels with it even though it is a pure function of position: a client deriving it would be a second implementation of the gate, free to drift from the server that enforces it.