Task 312: Cross-validate mass_timeline Tsiolkovsky Functions

Status: DONE

Description

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

• propellant_consumed (Tsiolkovsky equation)
• post_burn_mass
• compute_timeline (multi-event timeline)
• total_propellant_consumed / propellant_margin

These use Tsiolkovsky's rocket equation and don't depend on ephemeris.