Use full windows for multi-year return metrics

[Whning0513]

Summary

• use exact 3-year and 5-year cutoffs for the annualized return rows in reports.metrics()
• keep the 10-year row on the same relativedelta(years=...) pattern
• add a regression test that catches the previous one-month truncation

Closes #516

Testing

• python -m pytest -q tests/test_reports.py -k "metrics_basic_mode or metrics_full_mode or metrics_with_rf or full_three_and_five_year_windows"