Add US poverty target coverage contract

[MaxGhenis]

Summary

• add a machine-readable Arch coverage contract for US poverty/nonfiler target source families
• register the existing BEA NIPA pension contributions source package alias
• replace the stale PE-US-Data calibration target markdown with Populace-era hard-target, validation-only, and source-gap coverage

Tests

• uv run ruff check arch/source_package.py arch/targets/init.py arch/targets/us_poverty.py tests/test_us_poverty_target_coverage.py
• uv run pytest tests/test_arch_boundaries.py tests/test_arch_source_package.py::test_source_package_alias_compiles_soi_table_1_1_specs tests/test_us_poverty_target_coverage.py