Use rai: prefix for top-level RAI fields (validator literal-key match)

Inject 13 RAI fields into croissant.json (incl. all 6 NeurIPS-required: dataLimitations, dataBiases, personalSensitiveInformation, dataUseCases, dataSocialImpact, hasSyntheticData)

Initial benchmark release: 1,539 logged runs + Croissant metadata + claim manifest