Function insphere 

pub fn insphere(a: Point3, b: Point3, c: Point3, d: Point3, e: Point3) -> Sign

The 3-D in-sphere predicate: side of e w.r.t. the oriented sphere through a, b, c, d.

Sign convention (non-standard — see module docs): when a, b, c, d are positively oriented (super::orient3d::orient_3d > 0), returns Sign::Negative if e is inside the sphere, Sign::Zero if e is on it, Sign::Positive if outside. The sense is reversed for negative orientation (inside = Positive).

This is the public ladder entry point — it escalates from filtered to compensated to exact as needed, never returning an uncertain sign.