Task 308: Comms Module Cross-validation

Status: DONE

Description

Add cross-validation for the pure-math functions in comms.rs:

• light_time_seconds / light_time_minutes / round_trip_light_time
• free_space_path_loss_db
• CommFeasibility classification thresholds

These don't depend on ephemeris and can be validated independently.

Also fix cargo fmt issue from Task 306.