Function invertibility_density_p 

pub fn invertibility_density_p(n: u32, p: u64) -> f64

The invertibility density over GF(p): Π_{j=1}^n (1 − p⁻ʲ) = |GL(n,p)| / p^{n²}. As the field grows the density → 1 (fewer linear collisions); p = 2 is the densest-collision regime, the smallest constant φ(½) ≈ 0.28879.