[ZMQbg/1] can't be killed！！！

My notebook is running in the backgroud and I use command & to do that.
But when I close a kernel and kill the backgroud process and I use the command kill -9.
Then I find some process named [ZMQbg/1] which is runing, it can't be killed by any command,
and its ppid become init(1). so what should I do to kill it without reboot?

The issue names background notebooks, kernels, `kill -9`, and orphaned `[ZMQbg/1]` processes, but no repository files or tests. Start by reproducing the shutdown sequence and tracing which notebook or kernel process owns the surviving process. Done means explaining the process lifecycle and providing a verified way to clean it up without rebooting.

Written by the indexing model from the issue text.