Nominalise employee income for wage taxes

[agurgone]

Summary

• pass current CPI into central-government tax computation
• treat employee_income as net employee income in base-period units
• nominalise employee income with CPI, then gross it up before computing income tax and employee/employer social-insurance taxes
• add a regression test covering CPI nominalisation and the net-to-gross wage tax base

Verification

• /Users/andone/Documents/python_projects/INET-consumption/.venv/bin/ruff check macromodel/agents/central_government/central_government.py macromodel/country/country.py tests/test_macromodel/unit/test_agents/test_central_government/test_central_government.py
• /Users/andone/Documents/python_projects/INET-consumption/.venv/bin/pytest tests/test_macromodel/unit/test_agents/test_central_government/test_central_government.py tests/test_macromodel/unit/test_country/test_country.py -q

Notes

Confirmed on clean main that employee_income is refreshed by the wage setter before taxes, but the wage setter does not CPI-adjust it. Firms and individuals nominalise the same employee-income base with CPI; central-government wage taxes did not.

Independent review then identified a second issue: employee_income is net of employee-side labour taxes, while central-government tax revenue should be computed from the corresponding gross nominal wage base. This PR now applies both transformations in tax computation: base-period net employee income -> nominal net employee income -> nominal gross employee income.