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_test.go

@@ -1,6 +1,7 @@
 package notify
 
 import (
+	"bytes"
 	"context"
 	"errors"
 	"net"
@@ -8,6 +9,38 @@ import (
 	"time"
 )
 
+func TestCloneBulkDedicatedSendRequestsDeepCopiesPayload(t *testing.T) {
+	src := []bulkDedicatedSendRequest{
+		{
+			Type:    bulkFastPayloadTypeData,
+			Seq:     1,
+			Payload: []byte("payload-a"),
+		},
+		{
+			Type:    bulkFastPayloadTypeReset,
+			Seq:     2,
+			Payload: []byte("payload-b"),
+		},
+	}
+	cloned := cloneBulkDedicatedSendRequests(src)
+	if len(cloned) != len(src) {
+		t.Fatalf("clone length = %d, want %d", len(cloned), len(src))
+	}
+	if &cloned[0] == &src[0] {
+		t.Fatal("request clone should not alias source slice elements")
+	}
+	if len(cloned[0].Payload) == 0 || len(src[0].Payload) == 0 {
+		t.Fatal("payload should not be empty")
+	}
+	if &cloned[0].Payload[0] == &src[0].Payload[0] {
+		t.Fatal("payload clone should not alias source bytes")
+	}
+	src[0].Payload[0] = 'X'
+	if bytes.Equal(cloned[0].Payload, src[0].Payload) {
+		t.Fatal("mutating source payload should not affect cloned payload")
+	}
+}
+
 func TestBulkDedicatedBatchPlainRoundTrip(t *testing.T) {
 	releasePayload, err := encodeBulkDedicatedReleasePayload(4096, 2)
 	if err != nil {