Function interpolate_powder_temp_curve 

pub fn interpolate_powder_temp_curve(curve: &[(f64, f64)], temp_c: f64) -> f64

Interpolate a muzzle velocity (m/s) from a measured powder-temperature curve at temp_c (Celsius). curve is (temperature_celsius, velocity_m_s) points; it is sorted ascending by temperature before use. Values below the first point or above the last are CLAMPED to the endpoint velocity (no extrapolation beyond measured data), and segments are linearly interpolated. A single point yields a constant.