Add CMS Medicaid monthly enrollment package

[MaxGhenis]

Summary

• adds the CMS Medicaid and CHIP monthly enrollment dataset source package from the April 2026 release
• emits December 2025 final state Medicaid/CHIP enrollment facts for total Medicaid+CHIP, Medicaid, CHIP, child, and adult enrollment
• wires the package alias and adds focused count/value/lineage tests beside the existing December 2024 CMS Medicaid package

Local validation

• uv run --python 3.13 ruff check arch/source_package.py tests/test_arch_source_package.py
• uv run --python 3.13 pytest -q tests/test_arch_source_package.py -k 'cms_medicaid_monthly_dataset or cms_medicaid_package_builds_december_2024_state_enrollment_facts or cms_medicaid_source_package_alias'
• uv run --python 3.13 arch validate-package cms-medicaid-chip-monthly-enrollment-dataset --year 2026
• uv run --python 3.13 arch build-suite cms-medicaid-chip-monthly-enrollment-dataset --year 2026 --out /tmp/arch-cms-medicaid-monthly-enrollment-dataset-2026 --replace

Build-suite output: 255 consumer facts, 10,608 source rows, 2,288 source cells, lineage coverage 1.0, 0 agent-acceptance errors.

Microplex check

Loaded /tmp/arch-cms-medicaid-monthly-enrollment-dataset-2026/consumer_facts.jsonl through Microplex's ArchConsumerFactJSONLTargetProvider. For model period 2025 it produced 51 state medicaid_total_enrollment targets from the CMS facts.