Function span_midpoint_error 

pub fn span_midpoint_error(
    surface: &NurbsSurface,
    pcurve: &NurbsCurve,
    edge: &EdgeRecord,
    forward: bool,
) -> Result<f64, String>

The same comparison as measure_edge_against_pcurve_image, sampled once at the midpoint of every knot span of the edge’s own 3D curve.

This is the anti-aliasing half described in the module doc: it samples where the curve is furthest from what it interpolates, by construction, and its sample set is derived from the curve’s own knots rather than from a fixed grid that a dyadically-subdivided curve can hide behind.

Exposed so the general pcurve → 3D image-curve transfer can floor its own self-verification with it without re-deriving the span walk.