Improve county-to-household mapping for ACA rating areas

[daphnehanse11]

1. Current state: Counties come directly from CPS GTCO variable (line 635 of cps.py) with no imputation

Proposed 2-phase approach:

1. Build PUMA-to-county crosswalk using Census relationship files + population-weighted allocation factors
2. Implement probabilistic county assignment for missing/suppressed data

Key data sources identified:

• Census PUMA-county files: https://www.census.gov/geographies/reference-files/time-series/geo/relationship-files.2020.html
• MCDC Geocorr tool: https://mcdc.missouri.edu/applications/geocorr.html