fix: 结果文件中 POC 漏洞只显示 vulnerable 不显示漏洞名称 (Closes #591)

POC 扫描结果存入 details["vulnerability_name"]，
但 TXT/CSV/NDJSON 三种输出格式只读 details["vulnerability"]，
key 不匹配导致漏洞名丢失，退化为显示 status 字段 "vulnerable"。
三种 writer 统一兼容两种 key。

common/output/stdout_writer.go

@@ -98,6 +98,9 @@ func (w *StdoutNDJSONWriter) flatten(r *ScanResult) *ndjsonRecord {
 	rec.Title = strVal(d, "title")
 	rec.URL = strVal(d, "url")
 	rec.Vulnerability = strVal(d, "vulnerability")
+	if rec.Vulnerability == "" {
+		rec.Vulnerability = strVal(d, "vulnerability_name")
+	}
 	rec.Username = strVal(d, "username")
 	rec.Password = strVal(d, "password")
 	rec.Plugin = strVal(d, "plugin")