Strengthen validation retries and checkpoint safety

internal/framework/pipeline/checkpoint.go

@@ -89,6 +89,7 @@ const (
 	CheckpointReasonReused                      CheckpointReasonCode = "checkpoint_reused"
 	CheckpointReasonAcceptedArtifactReused      CheckpointReasonCode = "accepted_artifact_reused"
 	CheckpointReasonRecomputeStep               CheckpointReasonCode = "recompute_step"
+	CheckpointReasonValidationIncompleteLineage CheckpointReasonCode = "validation_incomplete_lineage"
 )
 
 type CheckpointDecision struct {
@@ -160,6 +161,8 @@ func checkpointDecisionDetail(reasonCode CheckpointReasonCode) string {
 		return "accepted normalized artifact is reusable"
 	case CheckpointReasonRecomputeStep:
 		return "selected step requires execution"
+	case CheckpointReasonValidationIncompleteLineage:
+		return "checkpoint reuse is disabled by validation-incomplete input lineage"
 	default:
 		return "checkpoint decision"
 	}