Read one Subject's raw values off ctx.expressId. Callers may pass a
full FilterRule in place of a Subject — every rule kind's shape is a
structural superset of its Subject (SubjectOf<R> in rule-set.ts), so
the extra op/value/values fields are simply ignored here.
Read one
Subject's raw values offctx.expressId. Callers may pass a fullFilterRulein place of aSubject— every rule kind's shape is a structural superset of itsSubject(SubjectOf<R>inrule-set.ts), so the extraop/value/valuesfields are simply ignored here.