Add marketplace selected-plan PTC proxy variables

[daphnehanse11]

Context

policyengine-us-data PR 618 adds a bronze-vs-benchmark marketplace plan selection proxy during calibration and publish. To make those published inputs usable in rules-side analysis, policyengine-us needs tax-unit variables for the selected-plan ratio and derived used/unused APTC amounts.

Needed inputs

• selected_marketplace_plan_benchmark_ratio on TaxUnit (float, annual input)
• optional debug-only inputs if useful: selects_bronze_marketplace_plan, state_marketplace_bronze_probability, state_marketplace_bronze_to_benchmark_ratio

Needed derived variables

• selected_marketplace_plan_premium_proxy = slcsp * selected_marketplace_plan_benchmark_ratio, but zero when aca_ptc <= 0
• used_aca_ptc = min(aca_ptc, selected_marketplace_plan_premium_proxy)
• unused_aca_ptc = max(aca_ptc - used_aca_ptc, 0)

Important

• Do not change the meaning of aca_ptc; it should remain the maximum available benchmark-based credit, not the used credit.
• Add tests showing used_aca_ptc + unused_aca_ptc == aca_ptc and that a bronze ratio below 1 leaves some credit unused.

This should stay separate from any later decision about whether calibration actually targets these new proxy variables.