Add GMAT interop tests for ocaml-odm
GMAT R2026a-generated OEM traces for LEO (ISS-like, 7 days, 20x20 gravity
+ MSISE90 drag), GEO (3 days), and Molniya (e=0.74, 2 days). 11 tests
validate OEM parsing, metadata, radius/velocity physical bounds, and epoch
ranges across all three orbit regimes.