Function brachistochrone_accel 

pub fn brachistochrone_accel(distance: Km, time: Seconds) -> f64

Required constant acceleration for a brachistochrone (flip-at-midpoint) transfer.

Model: accelerate for half the distance, flip, decelerate for the other half. a_required = 4 * d / t²

Returns acceleration in km/s².

Assumption: straight-line path, constant thrust, no gravity, rest-to-rest.