Add KFF marketplace enrollment source package

[MaxGhenis]

Summary

• add the KFF full-year average marketplace effectuated enrollment source package
• pin the package to 2024 facts so the default Arch bundle produces stable 2024 ACA enrollment targets
• wire the source package alias and bundle/source-package coverage tests

Microplex impact

• increases PE-native broad target coverage from 47/189 to 49/189 after the TANF package
• covers the state ACA/PTC person-count targets from KFF marketplace effectuated enrollment
• leaves ACA PTC amount and return-count cells uncovered because those need direct total amount/return sources, not enrollment counts

Validation

• uv run ruff check arch db tests/test_arch_bundle.py tests/test_arch_source_package.py
• uv run pytest tests/test_arch_source_package.py::test_kff_marketplace_effectuated_enrollment_alias_validates_fixture_counts tests/test_arch_source_package.py::test_kff_marketplace_effectuated_enrollment_builds_2024_state_facts tests/test_arch_bundle.py::test_build_bundle_writes_merged_consumer_contract -q
• uv run pytest -q
• Microplex coverage probe: 49/189 covered