[Alt] Add TAXSIM compatibility variables in PE-US

[MaxGhenis]

Summary

• add namespaced taxsim_* compatibility variables in PE-US for ambiguous TAXSIM outputs
• keep the migration surface in PE-US rather than encoding those semantics primarily in policyengine-taxsim
• add focused tests for the compat layer and the OK/MN split-credit adapters

Notes

• This is a competing alternative to [codex] Clarify legacy TAXSIM umbrella vars #7898 and [codex] Move TAXSIM umbrella outputs into adapters policyengine-taxsim#793.
• The matching TAXSIM PR prefers these PE-US compatibility vars and keeps an explicit fallback for older policyengine-us releases.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (edaf7b1) to head (bfc4436).
⚠️ Report is 31 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #7899      +/-   ##
===========================================
+ Coverage   97.26%   100.00%   +2.73%     
===========================================
  Files           5         3       -2     
  Lines          73       134      +61     
  Branches        2         1       -1     
===========================================
+ Hits           71       134      +63     
+ Misses          1         0       -1     
+ Partials        1         0       -1     

Flag	Coverage Δ
unittests	100.00% <100.00%> (+2.73%)	⬆️

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[MaxGhenis]

Closing this alternative in favor of #7898, which matches the repo split we chose and is already green. Keeping the compat-layer approach here as a documented alternative if we revisit a migration-oriented PE-US compatibility surface later.