Add importer for policyengine-bundles releases

[anth-volk]

Fixes #388

Summary

• Add scripts/import_policyengine_bundle.py to import verified policyengine-bundles release assets into policyengine.py.
• Vendor one current exploded bundle under src/policyengine/data/bundle/ and regenerate the legacy country release manifests used by runtime code.
• Update country/core optional dependency pins from the imported bundle and write a Towncrier changelog fragment.
• Document the bundle-consuming flow in release-bundle guidance.

Validation

• ruff check .
• ruff format --check .
• PYTHONPATH=src python -m pytest --noconftest tests/test_import_policyengine_bundle.py