Add Duration.Human() method for localized, human-readable duration formatting:
- Adaptive granularity: displays 2 largest time units
- i18n support for EN and DE languages
- Handles singular/plural forms automatically
- Supports negative durations with minus prefix
- Zero duration special case handling
Performance: ~0.88µs/op (11x faster than 10µs target)
Coverage: 100% on diff.go, 98.2% overall
Files modified:
- diff.go: Added Human() method with fmt import
- diff_test.go: Added 18 comprehensive tests + 2 benchmarks
- example_test.go: Added 3 example functions