Function longitudinal_trend 

pub fn longitudinal_trend(
    series: &[TimePoint],
    forecast_days: f64,
    improvement_direction: i8,
) -> TrendResult

OLS linear regression over a biomarker time series. improvement_direction: 1 = rising is good (e.g. eGFR), -1 = falling is good (e.g. BP, HbA1c).