Function fpfh_descriptors 

pub fn fpfh_descriptors(
    cloud: &PointCloud,
    feature_radius: f32,
) -> Result<Vec<[f32; 33]>, SpatialError>

Computes an FPFH descriptor for every point in cloud.

The cloud must carry normals. feature_radius is the neighborhood radius used to build each descriptor (≈5× the point spacing is a good start). This is the reusable building block behind FpfhRansacRegistration; compute it once on a keypoint cloud to drive descriptor-based matching cheaply.