Add xclim-backed VPD support

[peetmate]

Summary

• add xclim-backed VPD helpers with explicit relative-humidity, dewpoint, and atlas-proxy paths
• surface VPD summaries through climate statistics and compare-period outputs
• add focused tests for VPD computation and propagated reporting

Testing

• .venv/bin/python -m pytest -q tests/test_vpd.py tests/test_statistics_source_policy.py tests/test_compare_periods_baseline.py tests/test_packaging_metadata.py tests/test_package_import_hygiene.py

Closes #8