Task 431: Close Remaining Rust Test Coverage Gaps

Status: DONE

Summary

Add tests for the 3 remaining untested public Rust functions:

1. ephemeris::mean_elements — direct validation of orbital elements for all planets
2. ephemeris::arrival_position — behavioral assertions for transfer arrival positions
3. comms::CommFeasibility::label_ja — verify Japanese labels for all variants

Rationale

• All other ~80+ public functions have test coverage
• These are pure functions, trivially testable
• Closes the last gaps in Rust core test coverage