fix(resolver): prevent double-close panic in DNSResolver.Stop()

Use atomic Swap instead of Load+close+Store to eliminate the race
window where concurrent Stop() calls could both pass the check.

internal/resolver/resolver.go

@@ -406,12 +406,11 @@ func (r *DNSResolver) doRefresh() {
 
 // Stop 停止解析器。
 func (r *DNSResolver) Stop() error {
-	if !r.started.Load() {
+	if !r.started.Swap(false) {
 		return nil
 	}
 
 	close(r.stopCh)
-	r.started.Store(false)
 	return nil
 }