Task 344: Cross-validate thrust profiles and Uranus ephemeris

Status: DONE

Description

Close the most significant cross-validation gaps:

1. Rust propagation with thrust profiles (ConstantPrograde, Brachistochrone) — validate against independent Python RK4 implementation with thrust
2. Uranus (and Neptune) ephemeris — validate planet_position for outer planets with independent Meeus implementation
3. Propagation state initializers (circular_orbit_state, elliptical_orbit_state_at_periapsis)
4. RK45 adaptive thrust propagation cross-check against RK4

Changes

• cross_validation_export.rs: +29 new exports (state initializers, ConstantPrograde RK4, Brachistochrone RK4, RK45+thrust, Uranus/Neptune J2000 + 2215 epoch)
• validate_supplementary.py: +29 checks with independent Python implementations (RK4 propagation with thrust, Meeus 3D orbital rotation for outer planets)
• Supplementary checks: 115→144, total cross-validation: 350→379
• Stats refresh: tasks 343→344, commits 482+→484+