Task 421: Add Unit Type Edge Case Tests

Status: DONE

Summary

The units.rs module has the lowest test-to-function ratio (0.53) in the crate. Several macro-generated types (Seconds, Mu, Radians arithmetic, KmPerSec arithmetic) lack individual tests. Edge cases for Km::abs(), KmPerSec::abs(), Radians::tan(), normalize_signed boundary, and Eccentricity::value()/Display are also missing.

Rationale

• Unit types underpin every calculation in the crate — silent breakage here cascades everywhere
• Macro-generated operators for Seconds and Mu have zero dedicated tests
• Radians::tan() not tested, normalize_signed boundary at exactly -π untested
• CLAUDE.md: TDD approach, all tests must pass