Cap self-employed pension contributions at the section 415(c) limit

[MaxGhenis]

Fixes #8389.

Summary

• add self_employed_pension_contribution_limit as the lesser of the section 415(c) annual additions dollar limit and modeled self-employment compensation
• add capped_self_employed_pension_contributions and route the self-employed pension ALD through it
• route Saver's Credit and Utah homeowner/renter relief nontaxable income through the capped self-employed pension value
• keep regular IRA contributions outside section 415(c), and avoid applying a person-wide annual-additions pool to unrelated 401(k)/403(b) elective deferrals
• narrow the DC PTC non-DC regression so it only asserts dc_ptc is zero outside DC, rather than asserting other states have no property tax credits

Verification

• uv run python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/household/expense/retirement/self_employed_pension_contribution_limit.yaml policyengine_us/tests/policy/baseline/gov/irs/credits/retirement_saving/savers_credit_qualified_contributions.yaml policyengine_us/tests/policy/baseline/gov/states/ut/tax/property/homeowner_renter_relief/ut_homeowner_renter_relief.yaml policyengine_us/tests/policy/baseline/gov/irs/income/taxable_income/adjusted_gross_income/above_the_line_deductions/self_employment_income.yaml -c policyengine_us
• uv run python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/gov/states/dc/tax/income/dc_ptc.yaml policyengine_us/tests/policy/baseline/household/expense/retirement/self_employed_pension_contribution_limit.yaml policyengine_us/tests/policy/baseline/gov/irs/credits/retirement_saving/savers_credit_qualified_contributions.yaml policyengine_us/tests/policy/baseline/gov/states/ut/tax/property/homeowner_renter_relief/ut_homeowner_renter_relief.yaml -c policyengine_us
• git diff --name-only upstream/main...HEAD | rg '\.py$' | xargs uv run ruff check && git diff --check upstream/main...HEAD
• git diff --name-only upstream/main...HEAD | rg '^(policyengine_us|changelog\.d)' | xargs uv run python policyengine_us/tests/run_selective_tests.py --coverage --files
• independent read-only review: implementation sound; added downstream cap regressions for Saver's Credit and Utah

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (92fdab8) to head (4f4fbca).
⚠️ Report is 68 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #8419   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         3    +1     
  Lines           57        39   -18     
  Branches         1         0    -1     
=========================================
- Hits            57        39   -18     

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

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