Add opt-in ACS congressional district age targets

[MaxGhenis]

Summary

• add opt-in ACS S0101 congressional district age targets to the US fiscal target compiler
• materialize CD population-age targets using normalized household district codes, including string-coded H5 columns like "0601"
• report the ACS CD source as active in source coverage only when --include-congressional-district-targets is enabled, while keeping ACS national/state age rows excluded

Validation

• uv run ruff format packages/populace-build/src/populace/build/us_runtime/fiscal_targets.py tools/build_us_fiscal_refresh_release.py packages/populace-build/tests/test_us_fiscal_targets.py packages/populace-build/tests/test_us_fiscal_refresh_builder.py
• uv run ruff check packages/populace-build/src/populace/build/us_runtime/fiscal_targets.py tools/build_us_fiscal_refresh_release.py packages/populace-build/tests/test_us_fiscal_targets.py packages/populace-build/tests/test_us_fiscal_refresh_builder.py
• uv run python -m pytest packages/populace-build/tests/test_us_fiscal_targets.py::test_acs_congressional_district_age_targets_are_opt_in packages/populace-build/tests/test_us_fiscal_refresh_builder.py::test_reviewed_exclusions_do_not_report_opted_in_acs_cd_source packages/populace-build/tests/test_us_fiscal_refresh_builder.py::test_population_age_targets_materialize_person_age_counts
• uv run python -m pytest packages/populace-build/tests/test_us_fiscal_targets.py packages/populace-build/tests/test_us_fiscal_refresh_builder.py packages/populace-build/tests/test_us_congressional_district_geography.py