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.

[MaxGhenis]

Reviewed 2026-07-08 as part of the open-PR sweep — disposition: stays open as an active draft. It's self-contained (new policyengine.scenarios namespace, nothing on main imports it), all checks green. Deliberately NOT merging while the CRFB engagement is held, for two reasons: (1) merging publishes a new public namespace (7 exported symbols) to PyPI as a compatibility surface; (2) the contract hardcodes a provisional calibration target (oact_2025_08_05_provisional + its sha256) that may change before the engagement finalizes. Pre-merge checklist when CRFB resumes: reconcile target_source/target_sha256 with the finalized OACT artifact, and make a deliberate call on whether policyengine.scenarios is the right public home vs keeping the contract engagement-side. Non-blocking nit: validate_metadata compares metadata['name'] against the contract's target_source field — confusing key/field naming.