Function fixed_reference_sweep_within 

pub fn fixed_reference_sweep_within(
    rings: &Rings,
    sample: impl FnMut(Scalar) -> GeomResult<SampledPath>,
    reference: Vec3,
    budget: Scalar,
) -> GeomResult<TriMesh>

fixed_reference_sweep whose walls stay within budget of the surface its rings trace along a smooth directrix (#231).

sample(b) samples the directrix to chord budget b; it is called again with a halved budget while the walls miss (see SampledPath). The rings’ own distance from a curved profile is the caller’s to add: the reference compiler flattens the profile to half its chord budget and passes the other half here.