Update PolicyEngine US to 1.691.12

[MaxGhenis]

Summary

• bump the exact PolicyEngine US data-build pin to the newly published policyengine-us==1.691.12
• refresh uv.lock
• add a changelog fragment

Why

PolicyEngine US 1.691.12 includes the CRFB non-refundable Social Security credit fix from PolicyEngine/policyengine-us#7959. This keeps policyengine-us-data current with the freshness guard merged in #990.

Tests

• env -u UV_FROZEN uv run python .github/scripts/check_policyengine_us_dependency.py --mode fail
• env -u UV_FROZEN uv run pytest tests/unit/test_publication_scripts.py -q
• env -u UV_FROZEN uv run --no-sync --with pyyaml python scripts/run_quality_guards.py
• env -u UV_FROZEN uv lock --locked
• git diff --check