1. Neither `area_id` nor `dataset`: lists every dataset (stat) StatsMapped
   tracks for one country ('ireland' or 'united-kingdom'), with its key,
   human label, and which geography levels it can be shown at. Ireland and
   the UK track genuinely different datasets -- call this first for the
   right country before assuming a stat_key exists there, to find the
   right `stat_key` for `compare`'s ranking mode.
2. `area_id` given, `dataset` omitted: lists every dataset available for
   that one area (e.g. "county:kerry" for Ireland, "uk:lad:e09000033" for
   the UK), with its latest figure, year-on-year change, and caveat labels
   only (not full caveat text -- use mode 3 for the full detail on any one
   dataset that matters). `area_id` comes from `list_areas`; `country`
   must match whichever country that call used, or this simply 404s
   ("unknown geography").
3. Both `area_id` and `dataset` given: full detail for one dataset in one
   area -- the latest figure, a written summary, full caveat text, and (if
   `history_months` is set) recent history. `dataset` is a `series_key`
   from mode 2's own response. `history_months` means actual months of
   history (0 = everything) -- e.g. 24 returns 2 years of an annual
   series, not 24 years. `country` must match `area_id`'s own country.

   `dataset` and `history_months` are only meaningful together with
   `area_id` (and, for `history_months`, `dataset` too, since it only
   applies to mode 3); giving either without its real precondition
   raises rather than silently dropping the argument and dispatching to
   the wrong mode.