Function time_dilation_constant_v 

pub fn time_dilation_constant_v(
    coordinate_time: Seconds,
    v: KmPerSec,
) -> Seconds

Time dilation: compute proper time (ship time) given coordinate time.

For a constant-velocity segment at speed v: Δτ = Δt / γ = Δt × √(1 - β²)

This is an approximation — real brachistochrone transfers involve continuously varying velocity. For a more accurate calculation, use brachistochrone_proper_time.