Since we introduced the userspace router, if I start the Firefox VM,
quit Firefox, wait for the VM to shut down, and then start it again,
networking no longer works, and I see repeating log lines like this:

2025-12-09 09:13:29.147240893  [2025-12-09T09:13:29Z WARN  spectrum_router::router] interface App(0) has been blocked for 1 sec, dropping packet

Full reproduction steps:

1. cd host/rootfs
2. nix-shell --run 'make clean && make run'
3. Start Firefox VM with vm-start
4. See that networking works
5. Quit Firefox and wait a few seconds for VM to stop
6. Start Firefox VM with vm-start again
7. No more networking, and log lines like the above