feat: -silent 模式输出 NDJSON 到 stdout，支持 AI agent 管道消费

- 新增 StdoutNDJSONWriter，silent 模式下每条扫描结果实时输出一行 JSON
- LogWithProgress 层拦截人类可读日志，绕过 logger sync.Once 初始化时序问题
- 支持 fscan -h xxx -silent | jq 管道用法

common/output/stdout_writer.go

@@ -0,0 +1,40 @@
+package output
+
+import (
+	"bufio"
+	"encoding/json"
+	"os"
+	"sync"
+)
+
+type StdoutNDJSONWriter struct {
+	mu     sync.Mutex
+	writer *bufio.Writer
+}
+
+func NewStdoutNDJSONWriter() *StdoutNDJSONWriter {
+	return &StdoutNDJSONWriter{
+		writer: bufio.NewWriter(os.Stdout),
+	}
+}
+
+func (w *StdoutNDJSONWriter) WriteResult(result *ScanResult) error {
+	w.mu.Lock()
+	defer w.mu.Unlock()
+
+	data, err := json.Marshal(result)
+	if err != nil {
+		return err
+	}
+	data = append(data, '\n')
+	if _, err := w.writer.Write(data); err != nil {
+		return err
+	}
+	return w.writer.Flush()
+}
+
+func (w *StdoutNDJSONWriter) Close() error {
+	w.mu.Lock()
+	defer w.mu.Unlock()
+	return w.writer.Flush()
+}