List contact fields

Lists core contact columns and custom field definitions under { data, pagination }. Use the field names in audience filters, contact search, and {{ field | fallback }} merge tags. Add ?include=coverage to decorate each row with fill stats — the % of contacts holding a non-empty value plus the most common values (sampled; approximate: true on large populations) — and optionally audienceId to scope those stats to one saved audience. Check coverage before personalizing on a field so most recipients see a real value, not the fallback.