Function serve_env 

pub fn serve_env<F>(status_fn: F) -> Result<Option<ControlServer>>
where
    F: Fn() -> ProcStatus + Send + 'static,

Bind $YAH_CONTROL_SOCK and answer status from status_fn.

Ok(None) when the variable is unset or empty — the process is not running under a supervisor that wants a channel, which is not an error. That is the contract that lets the same binary run unchanged outside a camp.

The closure runs on the listener thread, once per request. Keep it cheap and keep it panic-free: a panic there takes the channel down, which the supervisor reads as a process that stopped answering.