[REGRESSION] Networking for Firefox broken on second VM run

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