The one client store of learner state, read by the composition-root hooks
and written by the learner-store adapters.
Remarks
Module-level on purpose: every surface in the tab must agree. It is only
ever filled in the browser (seedLearnerStore refuses without a
window), so a server process never holds one learner's data while
rendering another's page.
The one client store of learner state, read by the composition-root hooks and written by the learner-store adapters.