Add passive partnership/S-corp income input to the NIIT base

[MaxGhenis]

The NIIT source list (gov.irs.investment.income.sources) included rental income but omitted the section 1411(c)(1)(A)(ii) passive-business leg entirely, so passive pass-through income was never taxed under NIIT.

This adds a passive_partnership_s_corp_income input — the section 469 passive subset of partnership_s_corp_income, which is already in gross income, so including the subset in the NIIT base adds no income to AGI — and appends it to the source list from 2013 (NIIT has included passive business income since inception). It defaults to zero, so behavior is unchanged until a dataset supplies it; the Microcosm data side can populate it from the JCT/SOI passivity evidence gathered in Microcosm #530.

Deliberately excluded: the estate-income leg of #9304. estate_income is not in gov.irs.gross_income.sources today (placeholder comments mark the spot), and taxing income under NIIT before it enters AGI would be incoherent — that leg is blocked on gross-income inclusion first, re-scoped on the issue.

Tests: above-threshold NIIT computation with the new input (3.8% × min(NII, excess AGI)), below-threshold zero, and a default-zero regression case; also deduplicates two identically-named existing test cases.

Refs #9304

🤖 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 (f7e7bd3).
⚠️ Report is 53 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##              main     #9306    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           12         1    -11     
  Lines          161        10   -151     
==========================================
- Hits           161        10   -151     

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

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[MaxGhenis]

Data-side population tracked in PolicyEngine/microcosm#722 (assignment stage: SCF non-actively-managed-business shape, SOI/Form 8960 level) with the anchor ingestion in PolicyEngine/chronicle#179. Until that lands this input stays at its zero default, so merging this PR changes no results — it makes the NIIT base able to receive the passive leg.