feat(notify): 重构通信内核并补齐 stream/bulk/record/transfer 能力

- 引入 LogicalConn/TransportConn 分层，ClientConn 保留兼容适配层
  - 新增 Stream、Bulk、RecordStream 三条数据面能力及对应控制路径
  - 完成 transfer/file 传输内核与状态快照、诊断能力
  - 补齐 reconnect、inbound dispatcher、modern psk 等基础模块
  - 增加大规模回归、并发与基准测试覆盖
  - 更新依赖库

transfer_api.go

@@ -0,0 +1,87 @@
+package notify
+
+import (
+	"context"
+	"errors"
+	"io"
+)
+
+type TransferReaderAt interface {
+	io.ReaderAt
+	Size() int64
+}
+
+type TransferWriterAt interface {
+	io.WriterAt
+}
+
+type TransferCommitter interface {
+	Commit(context.Context) error
+}
+
+type TransferSyncer interface {
+	Sync(context.Context) error
+}
+
+type TransferCloser interface {
+	Close() error
+}
+
+type TransferDescriptor struct {
+	ID       string
+	Channel  TransferChannel
+	Size     int64
+	Checksum string
+	Metadata map[string]string
+}
+
+type TransferSendOptions struct {
+	Descriptor       TransferDescriptor
+	Source           TransferReaderAt
+	ChunkSize        int
+	Parallelism      int
+	MaxInflightBytes int64
+	VerifyChecksum   bool
+}
+
+type TransferReceiveOptions struct {
+	Descriptor       TransferDescriptor
+	Sink             TransferWriterAt
+	SyncOnCheckpoint bool
+	VerifyChecksum   bool
+}
+
+type TransferAcceptInfo struct {
+	Descriptor          TransferDescriptor
+	LogicalConn         *LogicalConn
+	TransportConn       *TransportConn
+	TransportGeneration uint64
+}
+
+type TransferHandle interface {
+	ID() string
+	LogicalConn() *LogicalConn
+	TransportConn() *TransportConn
+	Snapshot() (TransferSnapshot, bool)
+	Wait(context.Context) error
+	Abort(context.Context, error) error
+}
+
+var (
+	errTransferIDEmpty              = errors.New("transfer id is empty")
+	errTransferSourceNil            = errors.New("transfer source is nil")
+	errTransferSinkNil              = errors.New("transfer sink is nil")
+	errTransferHandlerNotConfigured = errors.New("transfer handler is not configured")
+	errTransferSizeInvalid          = errors.New("transfer size must be non-negative")
+	errTransferSessionNotFound      = errors.New("transfer session not found")
+	errTransferSessionExists        = errors.New("transfer session already exists")
+	errTransferStreamAlreadyActive  = errors.New("transfer stream already active")
+	errTransferSegmentOffset        = errors.New("transfer segment offset mismatch")
+	errTransferSizeMismatch         = errors.New("transfer size mismatch")
+	errTransferChecksumMismatch     = errors.New("transfer checksum mismatch")
+	errTransferChecksumUnsupported  = errors.New("transfer checksum verification requires io.ReaderAt")
+	errTransferSequenceEncodeNil    = errors.New("transfer sequence encoder is nil")
+	errTransferSequenceDecodeNil    = errors.New("transfer sequence decoder is nil")
+	errTransferAlreadyCompleted     = errors.New("transfer already completed")
+	errTransferAlreadyAborted       = errors.New("transfer already aborted")
+)