Function fixed_reference_sweep_within 

pub fn fixed_reference_sweep_within(
    rings: &Rings,
    sample: impl FnMut(f64) -> Result<SampledPath, GeomError>,
    reference: DVec3,
    budget: f64,
) -> Result<TriMesh, GeomError>

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.