Function register 

pub fn register(
    command: &str,
    model: Option<&str>,
    backend: &str,
    policy: InstancePolicy,
) -> Result<InstanceGuard, InstanceConflict>

Registers this process and enforces policy.

Registration happens before the conflict check, so two processes racing each other both see the other and both refuse. That is the safe direction: a spurious refusal costs a retry, a spurious admit costs a thrashed host and two meaningless measurements.