Get demographic breakdown for an audience

Returns a computed demographic breakdown for the audience, evaluated over the audience as a composition: its include-role segments combined per set_logic, minus any excluded segments. Works for every composition — pure-filter, a single non-filter subtype (matched, similarity, propensity), or a genuine mix — via a materialized scratch id-set bridge for compositions that include a non-filter segment. Response is cached server-side for 20 minutes. A population or individual demographic cell below the 50-record privacy floor returns 422 without a partial breakdown. Requires 'discovery' scope.