fix(risk): enforce paired scenario envelopes

[Pigbibi]

Summary

• compare every strategy path with its paired benchmark envelope so a severe stress baseline cannot relax OOS or bootstrap constraints
• weight WALK_FORWARD, BOOTSTRAP, and STRESS evidence families equally instead of letting bootstrap replica count dominate
• report selected-path observed drawdown rather than collapsing scenario envelopes into one loose scalar
• document the v2 extension boundary for strategies, portfolios, DCA, and plugins

Validation

• python -m unittest discover -s python/tests -v (237 tests passed)
• python -m py_compile python/scripts/long_horizon_risk_composer.py python/tests/test_long_horizon_risk_composer.py
• git diff --check

No strategy parameter, runtime target, policy write, or execution change is included.