Function detect_surface_kind 

pub fn detect_surface_kind(surface: &NurbsSurface) -> DetectedSurfaceKind

Detect the geometric kind of a NURBS surface by sampling.

Samples an 8x8 grid and checks for sphere (equidistant from centroid) or cylinder (equidistant from a PCA axis). Falls back to BSpline.

This is a lightweight heuristic — use recognize_surface for full analytic parameter recovery.