Function deviation_per_m 

pub fn deviation_per_m(
    balance: u64,
    limit_lower: u64,
    limit_upper: u64,
    lower_skew_offset_bps: u16,
    upper_skew_offset_bps: u16,
) -> Result<i32, CoreError>

Map inventory balance to signed deviation in ppm (−1e6..=+1e6). Dead zone → 0; outer limits → ±1e6.