WinRacing V51.8 Backtest + Audit Release
- Added read-only racing_results backtest adapter.
- Added banker Win/Top2/Top3/Top4 audit.
- Added action/tier/exposure result validation.
- Added exposure Top3 capture metric.
- Added release readiness audit trace.
- V50 source and V44-V50 scoring remain unchanged.
- No schema-changing SQL.