Module
What progress accounting needs of a lesson: which lesson it is, and how long it runs. Any ModuleLesson satisfies it.
What a course overview needs to know about one module without opening it:
how many lessons it holds, how long they run in total, and every lesson in
sequence order — enough to preview the module's artwork, count how far the
learner has got across all of it, and name whichever lesson comes next.
One lesson as the course overview sees it: its position in the module, its title, its runtime and its artwork, and nothing more.