Task 321: Priority Cross-Validation Additions

Status: DONE

Description

Add cross-validation for high-priority uncovered functions:

• orbits: elements_to_state_vector, specific_energy, specific_angular_momentum, mass_flow_rate, jet_power
• kepler: true_to_eccentric_anomaly, eccentric_to_mean_anomaly, propagate_mean_anomaly
• flyby: heliocentric_exit_velocity
• comms: planet_light_delay, distance_between_positions