Add Medicaid and CHIP spending targets to the US fiscal surface

Context

The healthcare target surface covers ACA Marketplace enrollment/APTC, Medicaid enrollment, Medicaid+CHIP combined enrollment, direct CHIP enrollment via #139/#165, and Medicare Part B premium income. It does not yet carry Medicaid/CHIP spending as a target cluster.

The code already treats weight-dependent Medicaid spending cautiously: test_weight_dependent_medicaid_spending_is_validation_only asserts that a CMS NHE Medicaid Title XIX expenditure fact is not promoted into the fiscal target registry. That keeps the current release safe, but it also means the healthcare tab cannot show Medicaid/CHIP spending fit.

Goal

Decide and implement the Populace-owned target/diagnostic contract for Medicaid and CHIP spending.

Suggested implementation

• Inventory source facts currently available from CMS NHE or other CMS Medicaid/CHIP expenditure sources.
• Decide which spending concepts can be hard targets, validation-only diagnostics, or intentionally excluded because the current PolicyEngine-US variables are formula-owned or unsupported.
• If hard targets are feasible, add target roles for Medicaid spending and CHIP spending separately where source support exists.
• If hard targets are not feasible yet, add explicit release diagnostics that show source totals and explain why they are validation-only.

Acceptance criteria

• Release artifacts make Medicaid and CHIP spending status visible instead of silently omitting the cluster.
• Target metadata distinguishes Medicaid, CHIP, and combined Medicaid+CHIP spending when the source supports that split.
• Tests cover the chosen behavior, including unsupported/validation-only facts.
• The healthcare diagnostics page can tell whether Medicaid/CHIP spending is present, absent by design, or blocked on model/source support.

Related:

• Direct CHIP enrollment: #139 / #165
• Medicaid and CHIP eligibility-to-enrollment diagnostics: #170

Start with test_weight_dependent_medicaid_spending_is_validation_only and inventory the CMS NHE or other CMS Medicaid/CHIP expenditure facts available to the project. Review the release artifacts and healthcare diagnostics page to determine whether Medicaid, CHIP, and combined spending can be hard targets or must remain validation-only; done means the chosen status is visible, metadata distinguishes supported concepts, and tests cover unsupported facts.