Fix 2026 QBID minimum to use active-business QBI

[saisharan0103]

Issue #9150 reports that the 2026 §199A(i) $400 minimum can apply based on ordinary QBI inputs even when there is no modeled active qualified trade or business income.

Root cause: qualified_business_income_deduction used the tax-unit sum of qualified_business_income plus sstb_qualified_business_income as the threshold base for the 2026 floor. Those are ordinary §199A deduction inputs and can be supplied directly, allowing the new minimum to override an otherwise zero wage/UBIA-limited deduction.

This changes the floor threshold to use a narrow Person-level active_qualified_business_income input documented as aggregate QBI from active qualified trades or businesses. The ordinary QBID computation and taxable-income cap are left unchanged.

Tests update the existing 2026 floor cases to provide active_qualified_business_income when the minimum is intended to apply, and add the reported above-phaseout zero wage/UBIA scenario showing ordinary QBI alone does not trigger the floor.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (838be3f) to head (7819321).
⚠️ Report is 14 commits behind head on main.

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Lines           31        31           
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  Hits            31        31           

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unittests	100.00% <100.00%> (ø)

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[MaxGhenis]

Cross-family dual review (Claude Fable 5 + GPT-5.6-sol, independent). Removing #9150's false positive is confirmed — the reported above-phaseout/zero-wage case now correctly returns $0, and pre-2026 results are mathematically unchanged. I pushed the missing changelog fragment. Blocked on the shape of the fix:

1. The floor goes dark on every normal data path. active_qualified_business_income has no formula, producer, or default, and nothing populates it — a live branch sim with ordinary qualified self-employment income of $1,000 gets active_qualified_business_income = 0 and loses the $400 minimum it legally qualifies for. The false positive became a universal false negative for modeled active businesses. 2026 QBID applies the §199A(i) $400 minimum to all QBI, not active-business QBI #9150 allowed either a distinct input or a derived aggregate; this needs the derived default (e.g. formula defaulting to ordinary QBI, overridable) or an explicit maintainer-approved required-input contract plus a data-seeding plan.
2. Person-level allocation breaks for floor-only cases. qualified_business_income_deduction_person allocates by qbid_amount shares; when the floor is the only deduction the unit gets $400 and every person gets $0. Needs handling + a test.
3. The fixtures assert an unverified statutory conclusion. Cases where the 20%-of-taxable-income cap is $120 expect $400 — i.e., §199A(i) overriding §199A(a)(2)'s overall limit. That may be right, but it's load-bearing and uncited; please add the statutory basis. One fixture also sets qbid_amount: 2,000 against total QBI of $1,000, an impossible ordinary §199A relationship.
4. No positive test reaches the floor through ordinary inputs — every passing case injects the new variable directly, which is what masks finding 1.