[codex] Recompute future SPM thresholds

[MaxGhenis]

What changed

• stop treating spm_unit_spm_threshold as a generic uprated input in future years
• add a shared SPM reference-threshold helper and rebuild future thresholds from prior-year implied geography, current tenure, and current SPM-unit composition
• add regression tests covering composition changes, tenure changes, and explicit future-year overrides

Why

SPM thresholds should not just be carried forward by CPI on the total stored amount. That loses the structure we built this for: the reference threshold, the equivalence scale, and the unit-specific geographic adjustment.

Impact

Future-year SPM thresholds in policyengine-us now respond to household composition and tenure changes while preserving the implied geographic adjustment from the stored base-year threshold.

Root cause

spm_unit_spm_threshold was defined as an input variable with gov.bls.cpi.cpi_u uprating, so future periods inherited a simple scalar uprate of the total threshold instead of recomputing the threshold components.

Validation

• PYTHONPATH=. pytest -q policyengine_us/tests/policy/baseline/household/income/spm_unit

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (20b8599) to head (d806fb0).
⚠️ Report is 38 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #7943   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            1         1           
  Lines           17        32   +15     
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+ Hits            17        32   +15     

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

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