Splines backed by rational NURBS (circles, conics) were tessellated as
uniform B-splines, losing the weighting that defines the curve shape.
Now BSplineCurve<Point3> is used only when weights are absent or empty;
when weight data is present a NurbsCurve<Vector4> is built from the
homogeneous control points so circles and conics render accurately.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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