Improves pure-Python ECI/ECEF accuracy

[ryanpavlick]

Summary

Improves the non-AstroPy ECI/ECEF path by replacing the old single sidereal-rotation fallback with a more complete Earth-orientation model.

What changed

• adds an internal Earth-orientation helper module for:
  • UTC normalization
  • embedded leap-second handling
  • UTC->TT conversion
  • optional UT1-UTC support
  • precession, nutation, apparent sidereal time, and polar motion
• upgrades the pure-Python eci2ecef() / ecef2eci() fallback to use that model
• threads optional delta_ut1, xp, and yp through higher-level wrappers
• improves sidereal-time handling in the pure-Python path
• tightens ECI/ECEF test expectations to reflect the new accuracy
• adds an EOP roundtrip test

Accuracy and performance

On the existing sample case, the pure-Python fallback error drops from kilometer-scale to meter-scale relative to Astropy.

This does make the fallback slower than before because it now computes leap-second handling, TT/UT1-related quantities, precession, nutation, sidereal rotation, and optional polar motion instead of a single sidereal rotation.

On local benchmarks for the pure-Python fallback:

• scalar eci2ecef() calls were about 12.6x slower
• a 1000-point same-epoch vector case was about 3.0x slower

The tradeoff here is intentionally accuracy over raw speed for the non-AstroPy path.

Testing

All tests pass. Adds a new EOP roundtrip test and tightens existing ECI/ECEF fallback tests.