Function solve_squarefree 

pub fn solve_squarefree(
    equations: &[ModpEquation],
    num_vars: usize,
    m: u64,
) -> Option<ModmOutcome>

Decide a linear system over ℤ/m for squarefree m, via CRT over the prime fields. Returns None only when m is not squarefree (the prime-power ring is a separate rung). The first prime factor whose system is inconsistent is reported as the witness — by CRT, the whole composite system is then inconsistent.