What a traveller must do to enter a destination

The eligibility engine, as procedures. One destination at a time — `filter[country]` is required, because a procedure is a rule for entering *a* country and all four under one `data` member would be four different answers.

**`exempt` is the only reason a nationality is absent from `data`**, and `meta.excluded.exempt` says how many were omitted. An exempt nationality has nothing to do, so it has no procedure — and the count is what keeps an empty result from being read as "we did not look".

**`filter[activeDate]` and `filter[originCountries]` are refused, not ignored.** We hold no rule validity windows and none of these destinations keys a rule on origin; the 400 says so. Answering a question about a date with rules that are not date-aware is the failure this engine exists to avoid.