Function parallel_lj_forces 

pub fn parallel_lj_forces(
    positions: &[[f64; 3]],
    epsilon: f64,
    sigma: f64,
    cutoff: f64,
) -> Vec<[f64; 3]>

Parallel Lennard-Jones (12-6) force computation.

For each particle i, accumulates contributions from all j within cutoff. The potential is U = 4*eps*[(sig/r)^12 - (sig/r)^6], giving F_i = sum_{j!=i} 4*eps [12(sig/r)^12 - 6(sig/r)^6] / r * r_hat_ij.

Interactions beyond cutoff are skipped.