Materialize Marketplace selected-plan benchmark ratios and metal-level diagnostics

Context:
Issue #70 found that selected_marketplace_plan_benchmark_ratio is 1.0 for every tax unit in the published Populace US 2024 H5. PR #73 declares selected-plan ratio construction in the aca_marketplace_inputs stage, but does not implement it.

Goal:
Generate Marketplace selected-plan benchmark ratios with real signal, then validate them against CMS metal-level Marketplace data.

Suggested implementation:

• For Marketplace take-up units, compute selected_marketplace_plan_benchmark_ratio from reported non-Medicare private premiums, assigned ACA PTC, and SLCSP.
• Clip or flag outliers using a reviewed support range, such as 0.5 to 1.5.
• Keep non-Marketplace units at the neutral default when the variable is not consumed.
• Add diagnostics comparing below-benchmark or bronze-like selections to CMS metal-level APTC enrollment targets where available.
• Record outlier counts and distribution summaries in the build diagnostics or release manifest.

Acceptance criteria:

• Marketplace takers have nonconstant selected-plan ratios.
• Unit tests cover ratio construction, clipping, zero/invalid SLCSP handling, and neutral defaults for non-takers.
• Diagnostics report state/national plan-choice shares against CMS metal-level targets.
• The health-input gate catches all-1.0 ratios before release.

Related:

• Umbrella: #70
• Source-stage declaration: #73
• Take-up materialization sibling: #74
• Degenerate-input guard: #71

Start at the aca_marketplace_inputs stage and review PR #73 alongside the take-up materialization in issue #74. Trace the reported premiums, assigned ACA PTC, and SLCSP inputs, then inspect the build diagnostics, release manifest, and health-input gate. Done means nonconstant Marketplace ratios, covered edge cases and neutral non-taker defaults, CMS metal-level comparisons, and outlier summaries.