checker: define Resolution verdict constants to make the collect/evaluate contract explicit

checker/types.go

@@ -8,6 +8,17 @@ import (
 
 const ObservationKeyDangling = "dangling_records"
 
+// Resolution verdict constants — the shared contract between Collect and Evaluate.
+const (
+	ResolutionOK       = "ok"
+	ResolutionNXDomain = "nxdomain"
+	ResolutionServFail = "servfail"
+	ResolutionNoAnswer = "no_answer"
+	ResolutionTimeout  = "timeout"
+	ResolutionError    = "error"
+	ResolutionSkipped  = "skipped"
+)
+
 // DanglingData is the raw observation payload; one Pointer per (owner, rrtype, target) triple.
 type DanglingData struct {
 	Zone            string    `json:"zone,omitempty"`