Add CRFB TOB scenario contract

[MaxGhenis]

Summary

• add a named CRFB Post-OBBBA long-run TOB scenario contract
• expose constants for the target source, scenario id, target hash, and trustees-2025 core threshold law mode
• add metadata validation helpers that reject raw current-law substitution and hash mismatches

Validation

• ruff check src/policyengine/scenarios tests/test_crfb_scenario_contract.py
• ruff format --check src/policyengine/scenarios tests/test_crfb_scenario_contract.py
• PYTHONPATH=src /Users/maxghenis/PolicyEngine/policyengine.py/.venv/bin/python -m pytest tests/test_crfb_scenario_contract.py

Notes

• This package carries only the portable scenario contract. It does not try to load policyengine-us-data or imply the long-term model/artifacts are installed.
• The first pytest attempt with PYTHONPATH=. failed because this repo uses a src layout; rerun with PYTHONPATH=src passed.