Deliberately NOT sf::st_read() on a GeoJSON string: that goes through GDAL and
then s2, which rejects a ring carrying a duplicate vertex — something a
coordinate-rounded coastline produces routinely, and something a planar cell
intersection does not care about at all. Reading the arrays directly also means
the fixture's geometry is exactly the numbers a TypeScript twin parses, with no
library in between to normalise them differently.