fix(notify): 根治低带宽控制面阻塞与传输写入卡死

- 为控制消息增加优先级、公平调度、队列字节预算和自适应批处理
- 支持可取消的写门等待，收紧 shared/dedicated bulk、stream 和 Reply 写入边界
- 修复 bulk reset/close、连接 handoff 和安全 profile 切换时序
- 保留旧取消与超时错误契约，新增阶段化 TransportSendError
- 增加 ReplyCtx、ReplyObjCtx、写超时配置及黑洞连接和竞态回归测试

transport_send_error.go

@@ -0,0 +1,75 @@
+package notify
+
+import (
+	"context"
+	"errors"
+	"fmt"
+)
+
+type TransportSendStage string
+
+const (
+	TransportSendStageQueue     TransportSendStage = "queue"
+	TransportSendStageWrite     TransportSendStage = "write"
+	TransportSendStageReply     TransportSendStage = "reply"
+	TransportSendStageTransport TransportSendStage = "transport"
+)
+
+type TransportSendError struct {
+	Stage TransportSendStage
+	Err   error
+}
+
+func (e *TransportSendError) Error() string {
+	if e == nil {
+		return "transport send error"
+	}
+	if e.Err == nil {
+		return fmt.Sprintf("transport send %s failed", e.Stage)
+	}
+	return fmt.Sprintf("transport send %s failed: %v", e.Stage, e.Err)
+}
+
+func (e *TransportSendError) Unwrap() error {
+	if e == nil {
+		return nil
+	}
+	return e.Err
+}
+
+func TransportSendErrorStage(err error) (TransportSendStage, bool) {
+	var sendErr *TransportSendError
+	if !errors.As(err, &sendErr) || sendErr == nil {
+		return "", false
+	}
+	return sendErr.Stage, true
+}
+
+func newTransportSendError(stage TransportSendStage, err error) error {
+	if err == nil {
+		return nil
+	}
+	var sendErr *TransportSendError
+	if errors.As(err, &sendErr) {
+		return err
+	}
+	return &TransportSendError{Stage: stage, Err: normalizeStreamDeadlineError(err)}
+}
+
+// publicContextSendError preserves the legacy public API sentinel when a
+// context cancellation is the cause of a transport send failure. Internal
+// callers still receive TransportSendError with its stage information.
+func publicContextSendError(ctx context.Context, err error) error {
+	if err == nil || ctx == nil {
+		return err
+	}
+	ctxErr := ctx.Err()
+	if ctxErr == nil {
+		return err
+	}
+	normalizedCtxErr := normalizeStreamDeadlineError(ctxErr)
+	if !errors.Is(err, ctxErr) && !errors.Is(err, normalizedCtxErr) {
+		return err
+	}
+	return normalizedCtxErr
+}