Returns the canonical set of values the country field of ETF geographic
allocation (/v2/etfs/countries) can take.

The list is derived from the ingest accept-list and is always consistent
with what the write pipeline will store. Each entry is tagged by type:

• country — ISO 3166 country/territory (incl. common aliases)
• region — multi-country grouping (e.g. "Europe", "Emerging Markets")
• supranational — non-sovereign issuer bucket ("Supranational",
  "European Union"), common in bond ETFs (World Bank, EIB, EU)
• other — non-geographic catch-all ("Cash", "Other", "Unknown", "N/A")

No ETF identifier is required.