Fix DC SE loss addition to cap at federally-deducted amount

[PavelMakarchuk]

Summary

• Caps dc_self_employment_loss_addition at the SE loss actually deducted in federal AGI via loss_ald, instead of using the raw self_employment_income
• When combined with the federal §461(l) excess business loss limitation (which caps deductible losses at $305K single / $610K MFJ in 2024), the old code created absurdly large positive DC AGI on taxpayers with large negative SE income

Example (from policyengine-taxsim#684)

Single filer, DC, self-employment income = -$25,000,000:

Metric	Before (bug)	After (fix)
Federal AGI	-$305,000	-$305,000
DC SE loss addition	$24,988,000	$293,000
DC AGI	$24,683,000	-$12,000
DC income tax	$2,635,878	$0

Root cause

dc_self_employment_loss_addition used raw self_employment_income to compute the addition. But federal AGI (DC's starting point) only contains the §461(l)-capped amount of SE loss. The addition should only add back what was actually subtracted.

Fix

Isolate the SE portion of loss_ald (which also includes capital losses) and cap the DC addition at that amount:

loss_ald = person.tax_unit("loss_ald", period)
limited_capital_loss = person.tax_unit("limited_capital_loss", period)
se_loss_in_ald = max_(0, loss_ald - limited_capital_loss)
effective_loss = min_(loss_taxunit, se_loss_in_ald)

Fixes #7408

Test plan

• All 5 YAML tests pass (2 existing + 3 new)
• Case 3: Large SE loss (-$25M) correctly capped at $293K addition (not $24.988M)
• Case 4: SE loss below federal cap but above DC threshold ($50K → $38K addition)
• Case 5: SE loss below DC threshold ($10K → $0 addition)
• Existing tests unchanged (losses below §461(l) cap are unaffected)

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (f073b57) to head (d3915d5).
⚠️ Report is 96 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #7437   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            4         1    -3     
  Lines           76        27   -49     
  Branches         1         0    -1     
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- Hits            76        27   -49     

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

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