fix: implement NJ same-category loss rule for gross income

[MaxGhenis]

Summary

• Implements New Jersey's "same category rule" (N.J.S. 54A:5-1) for gross income calculation
• Under this rule, if any income category has a net loss, that loss is disregarded (treated as $0) and cannot offset income from other categories
• Applies max_(0, category_total) to capital gains, partnership/S-corp income, self-employment income, farm income, and rental income

Fixes #7017

Test plan

• Added YAML tests covering the same-category loss rule scenarios
• Verified existing NJ integration tests still pass
• Ran make format to ensure code style compliance

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (67abb79) to head (c4d23f8).
⚠️ Report is 30 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##             main     #7280       +/-   ##
============================================
+ Coverage   72.11%   100.00%   +27.88%     
============================================
  Files        3794         1     -3793     
  Lines       54662        16    -54646     
  Branches      275         1      -274     
============================================
- Hits        39418        16    -39402     
+ Misses      15228         0    -15228     
+ Partials       16         0       -16     

Flag	Coverage Δ
unittests	100.00% <100.00%> (+27.88%)	⬆️

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