Assign SNAP take-up from state participation rates

[daphnehanse11]

Summary

• replace the national SNAP take-up top-up with FY2022 USDA FNS state participation rates for all 50 states and DC
• calibrate the SPM-unit flag against weighted eligible people (design weight × modeled snap_unit_size) while preserving reported SPM_SNAPSUB > 0 recipients as an anchor
• apply each state's calibrated stable-draw cutoff to currently ineligible units too, preserving a take-up response for eligibility-expanding reforms
• keep support-channel clones together, emit state/national participation diagnostics, and hard-gate missing rates, lost anchors, collapsed eligibility, state rate misses, and unexplained universal take-up
• move the stage after the SNAP eligibility inputs and SCF assets are materialized so PolicyEngine-US can supply is_snap_eligible and snap_unit_size

Source and semantics

The manifest now carries USDA FNS's Reaching Those in Need: Estimates of State SNAP Participation Rates in 2022, Table 1:

• landing page: https://www.fns.usda.gov/research/snap/state-participation-rates/2022
• report: https://www.fns.usda.gov/sites/default/files/resource-files/ear-snap-Reaching-Those-in-Need-2022.pdf

The estimates measure the share of eligible people participating in an average month. FNS publishes whole-percentage estimates and reports estimates at or above 100% as 100%; the manifest and diagnostics document that limitation.

Verification

• uv run pytest packages/populace-build/tests -q
• uv run ruff check ...
• uv run ruff format --check ...
• git diff --check

Closes #372