Function fit_helix_curve 

pub fn fit_helix_curve(
    axis_origin: Vec3,
    axis_direction: Vec3,
    reference: Option<Vec3>,
    start_radius: f64,
    end_radius: f64,
    pitch: f64,
    turns: f64,
    start_angle: f64,
    left_handed: bool,
) -> Result<NurbsCurve, String>

helix_sample_points fitted as ONE cubic curve (global interpolation — interpolate_curve, the machinery every fitted path here uses). Cubic interpolation error on a circle of radius R with node spacing Δθ is ≈ R·Δθ⁴/384: ~2·10⁻⁷·R at 64 stations per turn.