Map CPS farm income to farm operations

[MaxGhenis]

Summary

• Map CPS FRSE_VAL farm self-employment income to farm_operations_income instead of farm_income.
• Add a focused regression test for CPS personal income construction.
• Add a towncrier changelog fragment.

Companion model PR: PolicyEngine/policyengine-us#8156

Root cause

PolicyEngine distinguishes Schedule J-style farm_income from Schedule F farm operations income. The PUF pipeline already maps these separately, but the raw CPS builder was storing FRSE_VAL in farm_income, which makes CPS inconsistent with the model semantics and the PolicyEngine US SECA fix.

Tests

• uv run python -m pytest tests/unit/datasets/test_cps_income_variables.py -q
• uv run ruff check policyengine_us_data/datasets/cps/cps.py tests/unit/datasets/test_cps_income_variables.py
• uv run python -m pytest tests/unit/datasets/test_cps_income_variables.py tests/unit/datasets/test_cps_tax_unit_construction.py tests/unit/datasets/test_cps_file_handles.py -q