Add national CMS Medicaid enrollment aggregate

[MaxGhenis]

Summary

• add a United States aggregate row to the CMS Medicaid December 2024 enrollment source package by summing guarded state rows
• preserve source-cell lineage for the aggregate through range selectors and state-abbreviation guards
• update bundle/source-package tests for the five new national CMS Medicaid facts

Microplex impact

• fills the national Medicaid enrollment target coverage gap for person_count where medicaid > 0
• refreshed Microplex broad target coverage moves from 159/189 to 160/189 with the new CMS Medicaid artifact

Validation

• uv run ruff check tests/test_arch_source_package.py tests/test_arch_bundle.py
• uv run pytest tests/test_arch_source_package.py::test_cms_medicaid_source_package_alias_validates_fixture_counts tests/test_arch_source_package.py::test_cms_medicaid_package_builds_december_2024_state_enrollment_facts -q
• uv run arch validate-package cms-medicaid-chip-monthly-enrollment-december-2024 --year 2024
• uv run pytest tests/test_arch_bundle.py::test_build_bundle_writes_merged_consumer_contract -q
• uv run arch build-suite cms-medicaid-chip-monthly-enrollment-december-2024 --year 2024 --out /Users/maxghenis/CosilicoAI/microplex-us/artifacts/arch_cms_medicaid_national_coverage_20260528/cms_medicaid_y2024 --replace
• refreshed Microplex coverage report: /Users/maxghenis/CosilicoAI/microplex-us/artifacts/arch_cms_medicaid_national_coverage_20260528/pe_native_broad_2024_coverage_y2023_plus_stc_z1_medicaid_nhe_medicare_liheap.json