Function entity_id_expr 

fn entity_id_expr(
    id_columns: &[String],
    qualifier: Option<&str>,
    col: &dyn Fn(&str) -> Expr,
) -> Expr

The identity values in declared order (broad to narrow), escaped and joined.

The identifying column names are deliberately absent: the same identity reaches us under different names per source — a trace table’s service_name against a metric table’s job — and encoding them would split one entity into one per signal. entity_id_attrs carries the names beside the id, where they document its origin without dividing it.

col constructs the column reference (unqualified for registry branches, join-side-qualified for the calls derivation).