Implement ACA marketplace take-up input materialization

Context:
Issue #70 identified that the published Populace US 2024 H5 has degenerate ACA take-up input signal: takes_up_aca_if_eligible is true for every tax unit. PR #73 declares the missing aca_marketplace_inputs source stage, but does not implement the constructor.

Goal:
Materialize ACA marketplace take-up in Populace instead of inheriting a constant/defaulted column from the base H5.

Suggested implementation:

• Use CPS reported Marketplace/subsidized Marketplace coverage as true anchors at the tax-unit level.
• Assign simulated take-up to non-reported tax units using stable seeded tax-unit draws and the ACA take-up rate input.
• Calibrate the assignment to CMS Marketplace/APTC enrollment targets, preferably by state where support allows.
• Preserve reported true anchors during calibration.
• Keep the implementation independent of policyengine-us-data.

Acceptance criteria:

• The resulting H5 has nonconstant ACA take-up signal.
• A deterministic test covers reported anchors and stable seeded assignment.
• A calibration/diagnostic test covers matching a small synthetic CMS target.
• The US fiscal refresh health-input gate passes on a constructed fixture with realistic take-up variation.

Related:

• Umbrella: #70
• Source-stage declaration: #73
• Healthcare target mapping: #68
• Degenerate-input guard: #71

Start with the source-stage declaration in PR #73 and the related context in issues #70, #68, and #71. Trace how CPS anchors, seeded tax-unit assignment, ACA take-up rates, and CMS targets enter the H5, then define deterministic tests for anchor preservation, stable assignment, synthetic calibration, and the fiscal refresh health-input gate. Done means the constructed H5 has realistic nonconstant take-up variation and all acceptance checks pass.