Function build_cost_map 

pub fn build_cost_map(
    node: &CompositionNode,
    store: &(impl StageStore + ?Sized),
) -> HashMap<StageId, u64>

Walk a composition graph and build a map of StageId → declared_cents.

Only stages that declare at least one Effect::Cost { cents } appear in the map. Stages not in the store (e.g. RemoteStage) are ignored.