Connect SNAP asset test to SIPP-imputed liquid assets

[MaxGhenis]

Summary

• Connects snap_assets to spm_unit_cash_assets (which sums bank_account_assets, stock_assets, bond_assets from SIPP imputation)
• Previously snap_assets was an input variable defaulting to 0, meaning the SNAP asset test never bound in microsimulation
• Adds statutory reference to 7 USC 2014(g) and improves documentation

Context

This is part of a larger effort to properly model SNAP resource limits, particularly relevant for analyzing BBCE (Broad-Based Categorical Eligibility) elimination. Without this connection, eliminating BBCE only tests the gross income channel — but BBCE also waives asset limits, which is the dominant channel for most affected households.

Test plan

• Verify snap_assets returns non-zero values in microsimulation
• Check SNAP eligibility changes when asset test is binding
• Companion PR in policyengine-us-data improves asset imputation quality

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (f073b57) to head (8fdee5d).
⚠️ Report is 96 commits behind head on main.

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files            4         1    -3     
  Lines           76        10   -66     
  Branches         1         0    -1     
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- Hits            76        10   -66     

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

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