Persist diagnostics in reusable pipeline state

internal/framework/pipeline/checkpoint.go

@@ -279,6 +279,15 @@ type CheckpointArtifact struct {
 	ChunkRef     source.SourceRef
 	Artifact     contracts.SerializedArtifact
 	SchemaDigest string
+	Diagnostics  []CheckpointDiagnostic
+}
+
+// CheckpointDiagnostic stores a producer-local diagnostic alongside a
+// reusable artifact. The runner supplies the current run's origin when it
+// promotes this value into a diagnostic group.
+type CheckpointDiagnostic struct {
+	Diagnostic   contracts.ProducerDiagnostic `json:"diagnostic"`
+	ValidatorKey string                       `json:"validator_key,omitempty"`
 }
 
 type ExtractCheckpoint struct {