nx_step_nurbs.nx

nx_step_nurbs.nx -- RATIONAL B-SPLINE SURFACE evaluation from STEP (cadtwin P3a: the ROUND parts, exact). OpenCascade exports cylinders/holes as complex-instance rational B-spline surfaces, degree (1,3): linear along the axis x rational-CUBIC around -- weights (1,1/3,1/3,1) = an EXACT circle arc (NURBS circle). This organ parses the complex instance (control net 2x4 + weights) and evaluates the rational Bezier in fixed point: Bernstein Q12, weights Q20 (fx256 would corrupt 1/3 -> Q20 keeps radius error ~ppm), coords fx256 (1 unit = 3.90625 um). ENGINEERING-TWIN doctrine: dimensional truth first -- the gate measures roundness/radius/positions against the file's own declared values, in integer-exact teeth where the math is exact (t=0, 1/2, 1) and measured-um bounds where it rounds. Units: as1 declares SI_UNIT(.MILLI.,.METRE.) -> mm confirmed at runtime, never assumed. license_tier: ORIGINAL