The option set of a Choice question, plus how to read the answer back.
Holds three things: the rubric map that goes on the wire, and the pair of functions that turn the service's choice string into a value of A and back. Keeping the mapping next to the keys is what lets a Choice question answer in the caller's own type rather than in raw strings.
Build one with Options.strings for options only known at runtime, or derive it from an enum (see derives Options, added alongside the typed question builders).