Constant ENV_EVALUATE 

pub const ENV_EVALUATE: &str = "ATUNE_EVALUATE";

The environment variable naming the final-stage estimand a held-out evaluation computes: checkpoint or recipe.

Set only during Study::evaluate_checkpoints, where the child loads ENV_CHECKPOINT (the trial’s latest checkpoint) and evaluates it without training, and Study::replay_recipes, where it trains afresh following ENV_RECIPE. Absent for live trials and for a reevaluate replay.