nx_geo_eqarea.nx

nx_geo_eqarea.nx -- GEO-005 (other half): EQUAL-AREA cells, the fix for the geohash's CELL-SIZE weakness. A lat/lon grid (geohash) partitions latitude by equal Δlat, but a cell's GROUND AREA = Δlon * (sin(lat_hi) - sin(lat_lo)) shrinks by cos(lat) toward the poles -- so a "uniform" geohash grid actually has cells ~7x smaller near the poles than at the equator (density bias). The CYLINDRICAL-EQUAL-AREA fix: project lat -> y = sin(lat) and partition by equal Δy. Because the spherical area element is exactly dlon * d(sin lat), equal Δlon*Δsin rectangles have EQUAL ground area at every latitude. Composes the Hilbert ordering (nx_geo_hilbert) -> equal-area cells in Hilbert order = the S2/H3 cell-uniformity target. REUSES fx.nx CORDIC sin (no float, no reinvention). EXCEED is MEASURED: lat/lon band areas vary >2x (via real fx_sin) while equal-area bands are uniform.