Add SGP4 comparison, covariance, and station contact GMAT traces
New GMAT interop tests:
ocaml-sgp4:
- SGP4-to-SGP4 comparison (FAILS): same ISS TLE through GMAT SPICESGP4
and ocaml-sgp4 diverges by 43 km after 14 hours. Needs investigation
(likely WGS72/WGS84 gravity model mismatch).
- Station contacts OEM for LA ground station (3 days, pass prediction data)
ocaml-collision:
- Covariance propagation trace: state + 6x6 covariance evolution over 1 day
(SNC process noise, VNB frame). Enables future Pc validation with
known covariance.