Add marketplace_net_premium variable (Marketplace premium net of PTC)

[MaxGhenis]

Partially addresses #8095.

What

New variable `marketplace_net_premium` = `selected_marketplace_plan_premium_proxy − used_aca_ptc`.

The net out-of-pocket Marketplace premium a tax unit pays after the advance PTC is applied. Rules-based counterpart to the CPS-imputed Marketplace portion of `health_insurance_premiums_without_medicare_part_b`.

Why not wire into SPM MOOP yet

The SPM MOOP wrapper (#8103) swaps imputed Medicare Part B for rules-based Medicare Part B cleanly because there's a dedicated imputed variable (`medicare_part_b_premiums`) that can be subtracted.

For Marketplace, the CPS-imputed Marketplace net premium is commingled inside `health_insurance_premiums_without_medicare_part_b` along with employer-sponsored insurance premiums, small Medicaid/CHIP premium receipts, etc. We can't cleanly subtract the Marketplace share without either:

1. A per-household "takes up Marketplace" flag in the CPS imputation, or
2. A reform-delta-only application pattern (only add the difference between reform and baseline computed values).

Either is a us-data-side change. Filing the wiring as a follow-up once the isolation mechanism is in place.

Meanwhile

• `marketplace_net_premium` is available for consumers that want rules-based Marketplace net cost (cliff calculators, affordability analyses, reform studies). The variable moves with PTC reforms and benchmark changes.

Testing

3/3 tests pass: normal case, no-takeup case, PTC-exceeds-premium zero-floor case.

Test plan

• Local tests pass
• `make format` / `ruff check` clean
• CI passes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (05d5d97) to head (b688a93).
⚠️ Report is 36 commits behind head on main.

Additional details and impacted files

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  Coverage   100.00%   100.00%            
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  Files           18         1    -17     
  Lines          353        13   -340     
  Branches         4         0     -4     
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- Hits           353        13   -340     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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