Add guarded US area artifact release path

[MaxGhenis]

Summary

• add a deterministic Populace US area-artifact splitter for state and congressional-district H5s
• wire --include-area-artifacts into the US fiscal refresh release builder and release manifests
• enforce all-or-nothing current regional release coverage: 51 states plus 436 current congressional districts
• reject malformed regional support early, including stale keys like CA-53 and state/CD geoid mismatches
• extend the populace-data release contract so hand-built manifests cannot publish partial regional artifacts

Notes

This intentionally fails closed for the current CD-expanded support H5, which still carries the older 117th district set. The follow-up target-aging/crosswalk work is tracked in #205.

Verification

• uv run ruff format ...
• uv run ruff check --fix ...
• uv run --python 3.13 --package populace-build pytest packages/populace-build/tests/test_us_area_artifacts.py packages/populace-build/tests/test_us_congressional_district_geography.py packages/populace-build/tests/test_us_fiscal_refresh_builder.py packages/populace-build/tests/test_spec_only_country_packages.py -q
• uv run --python 3.13 --package populace-data pytest packages/populace-data/tests/test_contract.py -q
• uv run --python 3.13 --extra us --package populace-build python tools/build_us_area_artifacts.py --help
• git diff --check
• real-H5 preflight check: stale CD support fails early with Congressional district 53 is invalid for CA, which has 52 district(s).