Function roll_sum 

pub fn roll_sum(count: u32, base_angle_rad: f64, flow_roll_rad: f64) -> f64

Σ sin² Λ_k over count evenly spaced fins, where Λ_k is the angle from the lateral airflow to fin k (Niskanen 2009 eq. 3.51–3.53). The first fin is at base_angle_rad and the airflow at flow_roll_rad, both from x_B toward y_B. Three or more fins give exactly N/2 at any roll.