Risk tier is now derived from the worst validation issue severity and
the issues that drove it travel with the summary, so consumers can show
operators why a tier was assigned instead of an opaque level.
- Add RiskSummary.rationale (defaulted, backward-compatible) holding
human-readable '[code] message' lines from the validation report
- Pin severity -> risk mapping: ERROR->HIGH, WARNING->MEDIUM, INFO->LOW
- DefaultRiskAssessor folds the validation-driven level into the summary
kami