checker: rework target_resolvable to check existence (NOERROR) instead of A/AAAA

checker/rules_common.go

@@ -9,10 +9,9 @@ import (
 
 // Defaults are centralised so Definition's docs and runtime reads cannot drift.
 const (
-	defaultMaxChainLength          = 8
-	defaultMinTargetTTL            = 60
-	defaultRequireResolvableTarget = true
-	defaultAllowApexCNAME          = false
+	defaultMaxChainLength = 8
+	defaultMinTargetTTL   = 60
+	defaultAllowApexCNAME = false
 	defaultRecognizeApexFlattening = true
 
 	// hintKey is the CheckState.Meta key the HTML report reads to render the