Migrate ACA health-input gates after PE-US take-up contract changes

Context:
PolicyEngine-US PR #8352 proposes separating observed Marketplace coverage from simulated ACA take-up by introducing simulated_aca_take_up_if_eligible and deriving takes_up_aca_if_eligible from reported Marketplace coverage or simulated take-up. Populace PR #71 currently guards the present model surface: takes_up_aca_if_eligible and selected_marketplace_plan_benchmark_ratio.

Goal:
Once the PE-US ACA take-up contract lands, update Populace's source-stage declaration, release gates, and diagnostics to check/generate the correct inputs.

Suggested implementation:

• Change the aca_marketplace_inputs source-stage output from takes_up_aca_if_eligible to simulated_aca_take_up_if_eligible when that variable exists in the released PE-US surface.
• Keep reported Marketplace coverage as a CPS-carried person-level input, not a simulated take-up output.
• Update the health-input nonconstant gate to check simulated_aca_take_up_if_eligible, selected_marketplace_plan_benchmark_ratio, and reported Marketplace coverage support.
• Ensure Populace does not persist formula-owned takes_up_aca_if_eligible once PE-US owns it as a formula.
• Add compatibility notes/tests for the transition period if Populace must support both PE-US contracts temporarily.

Acceptance criteria:

• Populace writes/checks the model-side input variable that PE-US expects after #8352.
• Formula-owned ACA outputs are not persisted as raw dataset inputs.
• Health-input gates still catch constant take-up and all-1.0 plan ratios.
• Tests document the migration from observed-plus-simulated take-up to the new contract.

Related:

• Umbrella: #70
• Populace source-stage declaration: #73
• Populace health-input guard: #71
• PolicyEngine-US contract PR: PolicyEngine/policyengine-us#8352

Start with the aca_marketplace_inputs source-stage declaration in #73 and the health-input guard in #71, then inspect the PolicyEngine-US contract change in #8352. Trace the transition between reported Marketplace coverage, simulated take-up, and formula-owned outputs. Done means the gates and diagnostics match the released contract and tests document the migration and catch constant take-up or all-1.0 plan ratios.