Region codes move to three letters plus an index

Codes are now iad1, fra1, sin1 — an IATA code and an index — replacing the old us-east style. The old names keep working until 9 September 2026 and return a Deprecation header meanwhile.

This is the only breaking change we have shipped this year, and it is the sort we would rather not ship at all. The old scheme could not express two regions in one metro, and we now have that in three places.