Add Trustees core threshold reform

[MaxGhenis]

Summary

• add an explicit trustees-2025-nawi-v1 economic-assumption helper for long-run Trustees scenarios
• add trustees-2025-core-thresholds-v1, which applies that NAWI/payroll-cap path and then wage-indexes core ordinary federal tax thresholds after 2034
• keep Social Security benefit-tax thresholds unchanged

Validation

• ruff check policyengine_us/reforms/ssa policyengine_us/tests/policy/contrib/ssa/test_trustees_core_thresholds.py
• ruff format policyengine_us/reforms/ssa policyengine_us/tests/policy/contrib/ssa/test_trustees_core_thresholds.py
• PYTHONPATH=. /Users/maxghenis/PolicyEngine/policyengine-us/.venv/bin/python -m pytest policyengine_us/tests/policy/contrib/ssa/test_trustees_core_thresholds.py
• git diff --check

Notes

• This is an explicit scenario assumption, not default current law.
• Average-wage growth is owned by the NAWI projection helper; the threshold reform consumes gov.ssa.nawi rather than carrying its own wage-growth table.