`skyscraper/bn254-multiplier/src/utils`: tighter bounds

[recmo]

On 2026-03-17 in ba92f89 “Merge pull request #320 from worldfnd/xr/subred”:

tighter bounds

        assert!(x >= r);
        assert!(le256([0, 0, 0, x - r], U64_2P));
    }

    #[kani::proof]
    // TODO tighter bounds
    fn div_p_6b_underapprox() {
        let x: u64 = kani::any();
        let q = div_p_6b(x);

        let r = U64_P_MULTIPLES[q as usize][3];

From skyscraper/bn254-multiplier/src/utils.rs:223