Forgets the person's explicit choices (the page changed its pre-set, so the newest word is the page's).
The user's choice for a channel (key matched case-insensitively), or undefined when they made none.
Records the user's choice for a channel. undefined clears it (back to "no choice").
Runs action with writes ignored — used while pushing the page's pre-set into live channels.
The exposure-preference store the widget installs: an explicit choice (the person's, made in this call) wins, then the page's pre-set, then whatever the host's own store remembers. Choices are passed through to the host's store so a signed-in person's persisted preference still works.