pub fn tpi_lambda(t: &Tpi) -> ([BigRational; 3], BigRational)Expand description
TPI λ-construction (exact) via Cramer on the three plane equations
nᵢ·x = cᵢ, with nᵢ=(Bᵢ−Aᵢ)×(Cᵢ−Aᵢ), cᵢ=nᵢ·Aᵢ (un-normalised → all
polynomials, no sqrt). Cite: Attene 2020 §4. d=det3(n1,n2,n3), λ =
the Cramer numerators (column k replaced by (c1,c2,c3)).