Function network_killswitch_available 

pub fn network_killswitch_available() -> bool

Whether the network kill-switch is really available on this platform: Linux when the seccomp filter assembles (supported arch), macOS when /usr/bin/sandbox-exec exists, false everywhere else (Windows AppContainer is a follow-up). Used by mermaid self-test and the exec tool as a safe, fork-free probe — it installs nothing.