Adds ECI/ECEF state transforms

[ryanpavlick]

Summary

Adds ECI/ECEF state transforms on top of the improved pure-Python Earth-orientation model.

What changed

• adds eci2ecef_state() and ecef2eci_state()
• adds Astropy-backed state transforms using CartesianDifferential
• adds pure-Python state transforms using the current Earth-orientation matrix plus a numerical matrix-rate helper
• exports the new state-transform APIs at the package top level
• adds coverage for:
  • state transform accuracy against Astropy
  • pure-Python state roundtrips
  • a stationary ECEF point producing nonzero ECI velocity

Accuracy and performance

The pure-Python state transforms are internally self-consistent and roundtrip cleanly when both directions use the same model.

Against Astropy on representative cases, the current pure-Python path is approximately:

• ~20 m in position
• ~0.015 m/s in velocity

This branch currently builds on the open Earth-orientation accuracy work, so GitHub will show both commits until that earlier PR merges.

Testing

All tests pass. Adds new ECI/ECEF state-transform tests.