Task 420: Add Mass Timeline Edge Case Tests

Status: DONE

Summary

The mass_timeline.rs module has all public functions covered by at least one test, but several edge cases and boundary conditions are untested. Add tests for: zero initial propellant, out-of-order events, concurrent-timestamp events, over-jettison, zero-duration burns, low-Isp scenarios, and the infinite mass ratio overflow path.

Rationale

• Edge cases in mass calculations can silently produce incorrect results
• Some code paths (infinite mass ratio clamp, zero-propellant margin) are untested
• CLAUDE.md: "Mass timeline analysis" and "Track container jettison, propellant consumption"
• Robust edge case coverage prevents regressions during analysis expansion