Function split_jacobian_from_cubic 

pub fn split_jacobian_from_cubic(g: &[BigInt], p: &BigInt) -> SplitGenus2

Glue E₁: y² = g(u) with its reverse E₂: y² = ĝ(u) into the genus-2 curve C: y² = g(x²). g is a cubic given low→high [g₀, g₁, g₂, g₃]. The result carries a split Jacobian — validated by the quotient maps and by the Richelot ±-splitting being reducible (δ = 0).