Add CBO household income concepts

[MaxGhenis]

What changed

• add explicit CBO-style household income concepts in policyengine-us
• add before-tax, after-tax, and equivalized variants for benchmarking
• add an employer-sponsored insurance premium input and include it in CBO market income
• add targeted policy tests for the new concepts

Why

The benchmark tooling needed a named, reusable U.S. income concept that gets much closer to CBO's household income framework than raw household_market_income or household_net_income.

Impact

This gives downstream analysis code a stable variable stack for CBO-style comparisons instead of rebuilding ad hoc proxy formulas outside the model.

Validation

• PYTHONPATH=/Users/maxghenis/worktrees/policyengine-us-cbo-bridge /Users/maxghenis/PolicyEngine/policyengine-us/.venv/bin/python -m policyengine_core.scripts.policyengine_command test /Users/maxghenis/worktrees/policyengine-us-cbo-bridge/policyengine_us/tests/policy/baseline/household/income/household/cbo_household_income.yaml -c policyengine_us
• PYTHONPATH=/Users/maxghenis/worktrees/policyengine-us-cbo-bridge /Users/maxghenis/PolicyEngine/policyengine-us/.venv/bin/python -m policyengine_core.scripts.policyengine_command test /Users/maxghenis/worktrees/policyengine-us-cbo-bridge/policyengine_us/tests/policy/baseline/household/income/equiv_cbo_household_income.yaml -c policyengine_us
• ruff check policyengine_us/variables/input/employer_sponsored_insurance_premiums.py policyengine_us/variables/household/income/household/cbo_*.py

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (7f3ec13) to head (1076c15).
⚠️ Report is 365 commits behind head on main.

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files            2        14   +12     
  Lines           46       136   +90     
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+ Hits            46       136   +90     

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

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