Split charitable deduction AGI cap by organization type

[MaxGhenis]

Summary

• Splits the non-cash charitable deduction AGI cap by organization type per 26 USC 170(b)(1)
• Adds new input variable charitable_non_cash_donations_non_50_pct_orgs (defaults to 0 for backward compatibility)
• Adds parameter ceiling/non_cash_to_non_50_pct_org.yaml = 0.3 (30% AGI cap)
• Updates charitable_deduction.py to apply separate caps per org type, including proper interaction with OBBBA floor logic (2026+)
• 5 new test cases, all 15 tests passing
• Fixes pre-existing incorrect test comments (non_cash ceiling is 0.5, not 0.3)

Known simplifications (documented with TODOs)

26 USC 170(b)(1) defines 4 contribution categories with separate AGI caps:

1. (G) Cash to 50% limit orgs: 60% AGI (post-TCJA) -- modeled via ceiling.all
2. (B) Cash to non-50% limit orgs: 30% AGI -- not yet split from Basic prototype #1
3. (C) Capital gain property to 50% limit orgs: 30% AGI (or 50% with basis-reduction election) -- modeled at 50% (the election)
4. (D) Capital gain property to non-50% limit orgs: 20% AGI -- modeled at 30% using the general (B) limit

These simplifications are acceptable because:

• Microdata (CPS/PUF) doesn't distinguish these categories
• The 50%/30% split is the most impactful distinction
• The variable defaults to 0, preserving backward compatibility

Test plan

• All 10 existing charitable deduction tests pass unchanged
• 5 new tests cover: 30% cap binding, mixed org types, overall ceiling binding, 30% cap alone, floor+split interaction
• CI passes

Closes #6418

Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (182c387) to head (4ac84f6).
⚠️ Report is 42 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
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  Coverage   100.00%   100.00%           
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  Files            2         2           
  Lines           25        40   +15     
  Branches         0         1    +1     
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+ Hits            25        40   +15     

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

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