Map taxsim_pprofinc to SSTB self-employment income

[MaxGhenis]

taxsim_pprofinc (TAXSIM v28, SSTB income of the primary and secondary taxpayer) carried no formula and was labeled "Assumed zero" — so every emulator comparison told TAXSIM no SSTB income exists, while the SSTB slice also never reached taxsim_psemp (which reads self_employment_income, the non-SSTB variable). SSTB income simply vanished from TAXSIM inputs.

This maps taxsim_pprofinc = sstb_self_employment_income summed over the head and spouse per the v28 definition. No double-boxing (Feenberg's rule from the 2026-07-24 meeting): psemp already excludes the SSTB variable. With the Microcosm occupation-classified SSTB split (Microcosm #530), the QBID comparison becomes meaningful above the thresholds for the first time.

Tests: zero without SSTB income, head-only mapping, and head+spouse summation with dependent exclusion.

Fixes PolicyEngine/policyengine-taxsim#1141

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (8326a14) to head (b376f18).
⚠️ Report is 53 commits behind head on main.

Additional details and impacted files

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  Coverage   100.00%   100.00%            
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  Files           12         1    -11     
  Lines          161        12   -149     
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- Hits           161        12   -149     

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[MaxGhenis]

Closing without merge — Pavel is right that this targets the wrong layer. The TAXSIM mapping logic moved to the policyengine-taxsim emulator, whose current main already maps pprofinc/sprofinc → sstb_self_employment_income (variable_mappings.yaml, with the §199A(d)(3) phaseout note) and sums psemp+pbusinc into self_employment_income per Feenberg's spec that businc ≡ semp. The pe-us contrib/taxsim variables this PR edited are a legacy copy; my "assumed zero" finding described that stale layer, not the live emulator. No pe-us change needed.