Constant ENV_CHECKPOINT 

pub const ENV_CHECKPOINT: &str = "ATUNE_CHECKPOINT";

The environment variable carrying a checkpoint reference for the child to resume from.

Set only for a forked or resumed trial (population-based training): it is the parent’s — or, on a resume, the trial’s own — latest checkpoint reference, a run-directory path or artifact id the child restores from (e.g. oniro’s run_training_resume). atune never reads what it points at; it is a reference hand-off. Absent for an ordinary trial, so a program that never forks or resumes never sees it.