Add Universal Variable Lambert solver for computing transfer orbits
between any two planets. Enables pork chop plot generation as SQL:
SELECT dep_date, arr_date, lambert_c3(3, 4, dep_date, arr_date)
FROM generate_series(...) dep CROSS JOIN generate_series(...) arr;
New functions:
- lambert_transfer(dep_body, arr_body, dep_time, arr_time) → RECORD
Returns C3 departure/arrival (km^2/s^2), v_infinity (km/s),
time of flight (days), and transfer orbit SMA (AU).
- lambert_c3(dep_body, arr_body, dep_time, arr_time) → float8
Convenience: departure C3 only, NULL on solver failure.
The solver uses Stumpff functions for unified elliptic/parabolic/hyperbolic
handling, with Newton-Raphson iteration and bisection fallback.
Each solve is sub-millisecond; PARALLEL SAFE for batch computation.
All 11 regression tests pass.
