Add SPM geographic adjustment input

[MaxGhenis]

Summary

• Add an SPM unit geographic adjustment input variable.
• Calculate SPM thresholds from current composition, tenure, and geographic adjustment from 2015 onward.
• Preserve backward compatibility by inferring geographic adjustment from prior-year stored thresholds when no direct input is supplied.

Tests

• uv run ruff check policyengine_us/variables/household/income/spm_unit/spm_unit_geographic_adjustment.py policyengine_us/variables/household/income/spm_unit/spm_unit_spm_threshold.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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (623a65c) to head (1612f6f).
⚠️ Report is 138 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##              main     #8246    +/-   ##
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  Coverage   100.00%   100.00%            
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  Files           12         2    -10     
  Lines          173        52   -121     
  Branches         4         1     -3     
==========================================
- Hits           173        52   -121     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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