Add employer FUTA and state unemployment payroll taxes

[MaxGhenis]

Summary

• add employer-side FUTA variables and parameters, including credit reduction rates
• add employer-side state unemployment tax modeling with jurisdiction-specific state tax variables aggregated into a national variable
• add employer_payroll_tax and employer_cost_of_employment, plus an override input for employer-specific SUTA rates
• add Virgin Islands jurisdiction-mask support so defined_for = StateCode.VI works for territory-specific variables

Notes

• FUTA is modeled as the 0.6% effective rate plus jurisdiction-specific credit reductions from DOL data
• SUTA defaults use each jurisdiction's 2024 taxable wage base and a default rate from DOL new-employer rates or, where the published rule is employer-specific, the reported average tax rate on taxable wages

Testing

• uv run python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/gov/irs/payroll/unemployment -c policyengine_us
• uv run python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/gov/irs/payroll/employer_payroll_tax.yaml -c policyengine_us
• uv run pytest policyengine_us/tests/code_health/variable_names.py

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (b660201) to head (6e42726).
⚠️ Report is 174 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##              main     #7819    +/-   ##
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  Coverage   100.00%   100.00%            
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  Files            1        64    +63     
  Lines           19       707   +688     
  Branches         0         1     +1     
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+ Hits            19       707   +688     

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

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[DTrim99]

Fix lint and add changelog entry