Function eval_expression 

pub fn eval_expression(
    expressions: &str,
    configurator_json: &Value,
    source: &str,
) -> Result<f64, String>

One-shot: build an Env from a history’s expressions source + its configurator JSON, then evaluate source to a scalar. The small surface the engine-native sketcher’s live dimension-value edit uses (deliverable S5.0) — equivalent to Env::build(expressions, configurator_json)?.eval(source).