ported/log10_fn

pub fn log10_kernel(n: f64) -> Result<f64, WorksheetErrorCode> {
    if n <= 0.0 {
        return Err(WorksheetErrorCode::Num);
    }
    // Excel's LOG10 is the x87 `fldlg2` + `fyl2x` fused op; match it bit-exact.
    Ok(crate::ported::excel_numeric::excel_log10(n))
}