Improve county-to-household mapping for ACA rating areas

[daphnehanse11]

Fixes #454

[MaxGhenis]

@PolicyEngine is this superseded by #457

[policyengine]

Yes, this PR is superseded by #457, which was merged on January 1, 2026.

Why #457 supersedes this PR:

Both PRs address issue #454 (improving county-to-household mapping for ACA rating areas), but they take different approaches:

PR #455 (this PR):

• Created policyengine_us_data/geography/cd_county_crosswalk.py
• Builds a CD-to-county crosswalk with allocation factors
• Generates cd_county_crosswalk.csv and cd_county_mappings.json files

PR #457 (merged):

• Created policyengine_us_data/datasets/cps/local_area_calibration/county_assignment.py
• Created policyengine_us_data/storage/calibration_targets/make_county_cd_distributions.py
• Generates county_cd_distributions.csv with P(county|CD) distributions
• Integrated into the broader stacked dataset builder system for local area calibration
• Already merged to main and is part of a comprehensive local area calibration infrastructure

Both use similar Census block-level data (119th Congress BEF + 2020 Census population), but PR #457 provides a complete, integrated solution that's already deployed. This PR would introduce duplicate/conflicting functionality.

Recommendation: Close this PR as superseded by #457.