Use small Populace US bundle by default

[MaxGhenis]

Summary

• certify the smaller US Populace release populace-us-2024-f0af251-703bd81a565c-20260620T201958Z as the default policyengine.py US dataset
• pin the US model extra back to the release-compatible policyengine-us==1.729.0 while leaving policyengine-core==3.28.0 and UK pins unchanged
• make data-release certification synthesize the national region dataset template when an older Populace release manifest lacks explicit metadata.region_datasets
• regenerate the US TRACE TRO sidecar and update bundle/model/region tests

Rationale

The current default US Populace artifact is the 224,026-household / ~1.2 GB build. The latest small certified public Populace artifact is the 75,112-household / ~349 MB f0af251 release. This PR makes that smaller artifact the default again to reduce app/API load while preserving explicit bundle certification metadata.

Validation

• uv run python scripts/bundle.py check
• uv run ruff format --check src/policyengine/provenance/certification.py tests/test_certify_data_release.py tests/test_release_manifests.py tests/test_models.py tests/test_us_regions.py
• uv run ruff check src/policyengine/provenance/certification.py tests/test_certify_data_release.py tests/test_release_manifests.py tests/test_models.py tests/test_us_regions.py
• git diff --check
• uv run --extra models python - <<'PY' ... resolved policyengine-us 1.729.0, policyengine-core 3.28.0, policyengine-uk 2.89.2
• uv run --extra models pytest tests/test_bundle.py tests/test_release_manifests.py tests/test_models.py tests/test_us_regions.py tests/test_certify_data_release.py -q (139 passed, 1 warning)
• uv build
• wheel inspection: packaged manifest contains populace-us-2024-f0af251-703bd81a565c-20260620T201958Z and policyengine-us==1.729.0; wheel has 0 __pycache__ entries