Net the property tax credit out of the MO WFTC liability cap

[hua7450]

Summary

Fixes the Missouri Working Families Tax Credit's tax-liability cap to net out the (refundable) Missouri Property Tax Credit, matching Form MO-WFTC lines 7–10. Previously the cap used raw pre-credit tax, so PolicyEngine overpaid the WFTC to households eligible for both credits.

Fixes #9179

Legal basis

• Form MO-WFTC lines 7–10 (identical in all years): Line 7 = MO-1040 Line 36 tax; Line 8 = MO-1040 Line 42 (Form MO-TC miscellaneous credits) + Line 43 (property tax credit); Line 9 = max(Line 7 − Line 8, 0); Line 10 = min(Line 6 credit, Line 9). 2025 MO-1040 Instructions
• RSMo 143.177.3: the WFTC "shall be applied against the income tax liability imposed by this chapter after reduction for all other credits allowed thereon." https://revisor.mo.gov/main/OneSection.aspx?section=143.177&bid=49978&hl=
• RSMo 135.020: the PTC's refundable excess is measured against "the income tax reduced by other credits," so the PTC stays refundable and its total is unaffected by this ordering — only the WFTC surviving the cap changes. https://revisor.mo.gov/main/OneSection.aspx?section=135.020&bid=6437

Changes

• New variable mo_wftc_liability_cap = max(mo_income_tax_before_credits − mo_property_tax_credit, 0), with a comment noting MO-1040 Line 42 (Form MO-TC) is not modeled. It lives in credits/ next to mo_wftc.py with a WFTC-scoped name because the quantity exists only as Form MO-WFTC Line 9 — MO-1040 itself has no such line, so a generic income_tax/ name would invite reuse as a real return line.
• Used as the liability base in mo_wftc.py only. The mo_non_refundable_credits aggregate stays on mo_income_tax_before_credits: it reads the already-capped applied mo_wftc, so the aggregate equals the applied credit either way, and the Form MO-WFTC line 8 netting is WFTC-specific — a WFTC-specific aggregate base would wrongly suppress any future credit placed ahead of the WFTC in the ordered list. (The issue originally suggested changing both call sites; that was based on the incorrect assumption that the aggregate recomputes credits from their potentials.)
• mo_property_tax_credit remains in refundable.yaml (it is genuinely refundable per RSMo 135.020).
• Fixed the stale "the last reference above" comments in mo_income_tax.py and mo_income_tax_before_refundable_credits.py to name RSMo 135.020 explicitly and link the refundability citation with a #page=17 anchor.
• Relabeled mo_income_tax_before_refundable_credits to "Missouri income tax before refundable credits" — it previously shared the exact label "Missouri income tax" with mo_income_tax, leaving two indistinguishable variables in API/app metadata that differ by the full refundable PTC.

Test plan

• Two unit cases in mo_wftc.yaml: PTC exhausting liability → WFTC $0; PTC partially reducing the cap → WFTC $40.
• Two unit cases for the new mo_wftc_liability_cap variable (reduction and zero floor), under tests/.../credits/wftc/.
• Pinned integration regression from the issue's verified reproduction: disabled single renter, MO 2025, $18,000 wages, $9,600 rent → tax $17.17, PTC $446.49, WFTC potential $16.89, mo_wftc $0, net MO tax −$429.32 (previously PolicyEngine paid the full $16.89 of WFTC).
• Existing WFTC/credit-order tests unaffected (their households compute a $0 PTC).
• CI

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (0eec199) to head (743cf68).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

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  Files            2         5    +3     
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+ Hits            31        62   +31     

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

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[MaxGhenis]

Merging under the cross-family dual-review protocol (Claude Fable 5 + GPT-5.6-sol, independent reviews, both merge-ok with no blocking findings).

Verified: RSMo 143.177.3 ordering language confirmed verbatim against revisor.mo.gov; the aggregate-unaffected claim proven against ordered_capped_state_non_refundable_credits (applied credit A = min(W, max(T−P, 0)) ⇒ aggregate min(A, max(T,0)) = A); all 19 changed YAML cases executed and passed; label dedup checked against metadata consumers. Nonblocking notes: the refundable-WFTC reform's taxsim_state_eitc umbrella still keys on baseline mo_wftc (predates this PR), and the partial-overlap case could additionally assert the aggregate path.