Task 319: Extend Kepler + Ephemeris Cross-Validation Coverage

Status: DONE

Description

Add cross-validation for uncovered kepler and ephemeris functions:

• kepler: mean_to_true_anomaly, true_to_mean_anomaly round-trip, mean_motion
• ephemeris: phase_angle, next_hohmann_window
• orbits: specific_energy, elements_to_state_vector

These are mathematically non-trivial functions used throughout the analysis.