fix: 修复 dedicated bulk attach 竞态并优化 short write 补写路径

- 客户端 dedicated attach 回复改为精确读取单帧，避免 attach reply 与后续 NBR1 数据粘连后被误解析
  - 服务端 accepted attach 改为先 detach transport，再直接回 attach reply，随后立即切入 dedicated bulk read loop
  - transport 读循环在 stop 或 transport ownership 失效后不再继续上推已读数据，避免 handoff 后首包被旧 reader 吃掉
  - dedicated bulk record 写路径改为 full-write，消除 short write 导致的 invalid bulk fast payload
  - 优化 vectored write 补写策略：先尝试一次 writev，未写完时直接顺序补完剩余 buffers，减少重复 WriteTo 开销
  - 放宽 vectored write 能力识别，支持通过 UnwrapConn/WriteBuffers 命中 fast path
  - 修复 dedicated batch 排队路径 payload 复用问题，改为深拷贝 queued items
  - 补齐 dedicated attach、short write、payload clone、transport stop/handoff 等回归测试

bulk_dedicated_batch.go

@@ -165,8 +165,7 @@ func (s *bulkDedicatedSender) submitWriteBatch(ctx context.Context, items []bulk
 	if submitted, err := s.tryDirectSubmitBatch(ctx, items); submitted {
 		return err
 	}
-	queuedItems := make([]bulkDedicatedSendRequest, len(items))
-	copy(queuedItems, items)
+	queuedItems := cloneBulkDedicatedSendRequests(items)
 	return s.submitBatch(ctx, queuedItems, true)
 }
 
@@ -458,6 +457,20 @@ func (s *bulkDedicatedSender) stoppedErr() error {
 	return errTransportDetached
 }
 
+func cloneBulkDedicatedSendRequests(items []bulkDedicatedSendRequest) []bulkDedicatedSendRequest {
+	if len(items) == 0 {
+		return nil
+	}
+	cloned := make([]bulkDedicatedSendRequest, len(items))
+	for i, item := range items {
+		cloned[i] = item
+		if len(item.Payload) > 0 {
+			cloned[i].Payload = append([]byte(nil), item.Payload...)
+		}
+	}
+	return cloned
+}
+
 func bulkDedicatedSendRequestLen(req bulkDedicatedSendRequest) int {
 	return bulkDedicatedSendRequestLenFromPayloadLen(len(req.Payload))
 }