unproject

Convert a tile-local point at zoom z, tile column x, tile row y, and extent extent (almost always 4096) into a sea-level Position.

Web Mercator slippy-tile math: x maps linearly to longitude; y inverts through sinh because Mercator y is arcsinh(tan(lat)). The point may legitimately fall outside [0, extent] — MVT tiles include a small buffer (typically 64 px) of geometry that overlaps neighbours to avoid hairline gaps; we project anywhere.