Blend SIPP and SCF financial asset imputations

[MaxGhenis]

Summary

• applies a stable household-level 50/50 source-model draw between SIPP and SCF for overlapping bank, stock, and bond assets
• adds SCF-only balance-sheet leaves for certificates of deposit, savings bonds, retirement assets, insurance, managed assets, real estate, business equity, vehicle/student/other installment debt, mortgages, credit cards, and other debts
• computes net_worth directly from the final mixed-source component formula, with no net_worth_residual and no rescaling of resource-tested policy leaves
• validates that the SCF source provides every formula target needed for the component sum
• updates docs and tests for the component formula and household-level source draw

Refs #835.

Validation

• uv run pytest tests/unit/test_asset_imputation.py tests/unit/calibration/test_source_impute.py tests/unit/datasets/test_cps_file_handles.py
• uv run ruff check policyengine_us_data/utils/asset_imputation.py policyengine_us_data/datasets/cps/cps.py policyengine_us_data/calibration/source_impute.py tests/unit/test_asset_imputation.py tests/unit/calibration/test_source_impute.py tests/unit/datasets/test_cps_file_handles.py
• git diff --check

[MaxGhenis]

Replaced by in-repo PR #839 so the repository CI can run; fork PRs are rejected by check-fork.