Task 443: Consolidate Duplicated Rust Constants

Status: DONE

Summary

C_KM_S (speed of light) is defined in both relativistic.rs and comms.rs. AU_KM is defined in both ephemeris.rs and comms.rs. Consolidate into constants.rs and import from there.

Files to Change

1. constants.rs — add AU_KM and C_KM_S constants
2. relativistic.rs — remove local C_KM_S, use constants module
3. comms.rs — remove local C_KM_S and AU_KM, use constants module
4. ephemeris.rs — remove local AU_KM, use constants module