feat(risk): represent split-adjusted price basis

[Pigbibi]

Summary

• represent split-adjusted price-return benchmarks without claiming they are total return series
• preserve fail-closed behavior: the current generic composer parks this basis pending a dedicated comparable-benchmark composer
• document the distinction and add contract coverage

Validation

• rtk pytest python/tests/test_long_horizon_risk_observation_v2.py python/tests/test_long_horizon_risk_observation_ingress_v2.py