Function brachistochrone_peak_velocity 

pub fn brachistochrone_peak_velocity(distance: Km, accel_km_s2: f64) -> KmPerSec

Peak velocity at the midpoint of a brachistochrone transfer.

v_peak = a × t_half / √(1 + (a × t_half / c)²)

For non-relativistic cases, this simplifies to a × t_half.