A projectile is launched from horizontal, level ground with an initial speed \(v_0\) at an angle \(\theta\) above the horizontal, where \(0^\circ < \theta < 90^\circ\), and air resistance is negligible. The projectile travels along a parabolic trajectory until it returns to the ground. What is the ratio of the magnitude of the projectile's average velocity vector over the entire flight to the magnitude of its instantaneous velocity vector at the apex of its trajectory?