A parsed ORDER BY clause: a single sort key (expression), ascending
by default. Phase 7b widened this from “bare column name” to
“arbitrary expression” so KNN queries of the form
ORDER BY vec_distance_l2(col, [...]) LIMIT k work end-to-end. The
expression is evaluated per-row at execution time via eval_expr;
the simple ORDER BY col form still works because that’s just an
Expr::Identifier taking the same path.