pub fn lpi_lambda(l: &Lpi) -> ([BigRational; 3], BigRational)Expand description
LPI λ-construction (exact): the implicit point is (λx/d, λy/d, λz/d).
Line PQ ∩ plane RST: parametrise X = P + τ·(Q−P); on-plane gives
(P−R)·(SR×TR) + τ·(Q−P)·(SR×TR) = 0, i.e. n + τ·d = 0, so τ = −n/d
and λ = d·P − n·(Q−P) (the MINUS is load-bearing — + lands off the
plane; verified by tritri::edge_crossing_lpi_lies_exactly_on_the_plane).
qp=Q−P; sr=S−R; tr=T−R; pr=P−R; d=det3(qp,sr,tr); n=det3(pr,sr,tr).