Add county-crosswalked FNS SNAP data as a second congressional-district benchmark

[daphnehanse11]

Problem

CD-level SNAP validation currently has one lens: ACS S2201 district household estimates (#240, PR #241) — a survey estimate with its own sampling error. Calibrating or validating against a single noisy benchmark launders that noise into confident-looking district results.

Proposal

USDA FNS publishes county-level SNAP participation and issuance (bi-annual). Population-weighted county→CD crosswalk (Census blocks or MCDC geocorr) yields an administrative district benchmark independent of ACS sampling error.

1. Ledger-side: ingest the FNS county bi-annual file as a source package
2. Crosswalk counties to current-vintage CDs (the Carry the block-anchored US geography ladder as spine columns #277 block ladder / Translate old-vintage CD targets to current district geography #205 vintage translation machinery is directly reusable)
3. Add as a second validation_only family beside snap_local_proxy
4. Promotion rule: CD SNAP targets stay validation-only until the two independent benchmarks corroborate each other district-by-district; divergence localizes either thin raw support or a real modeling gap

Context

From the SNAP research readout follow-ups: congressional-district SNAP results should remain exploratory until independently benchmarked. This issue defines what 'independently benchmarked' means concretely.