- Add solve_barker_equation() function using cubic formula: D + D³/3 = M
- Integrate Barker's equation into propagate_orbital_elements() for parabolic orbits
- Add comprehensive test suite (11 tests, 239 assertions) following TDD
- Use cbrt() for cube root (handles negative numbers properly)
- Parabolic propagation now uses exact analytical solution instead of iterative solver
- All 93 tests passing (239,872 assertions)