Compute SPM geographic adjustments from geography

[MaxGhenis]

Summary

• make spm_unit_geographic_adjustment formulaic from household congressional district and SPM tenure
• use bundled spm-calculator congressional-district geoadj data
• remove the prior-year threshold backout behavior

Tests

• uv run ruff check policyengine_us/variables/household/income/spm_unit/spm_unit_geographic_adjustment.py policyengine_us/tests/policy/baseline/household/income/spm_unit/test_spm_unit_spm_threshold.py
• uv run pytest policyengine_us/tests/policy/baseline/household/income/spm_unit/test_spm_unit_spm_threshold.py

[codecov]

Codecov Report

❌ Patch coverage is 89.47368% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.59%. Comparing base (623a65c) to head (83aee8f).
⚠️ Report is 142 commits behind head on main.

Files with missing lines	Patch %	Lines
.../income/spm_unit/spm_unit_geographic_adjustment.py	89.47%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              main    #8249      +/-   ##
===========================================
- Coverage   100.00%   92.59%   -7.41%     
===========================================
  Files           12        1      -11     
  Lines          173       27     -146     
  Branches         4        2       -2     
===========================================
- Hits           173       25     -148     
- Misses           0        2       +2     

Flag	Coverage Δ
unittests	92.59% <89.47%> (-7.41%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.