Get demographic breakdown for a segment

Returns a computed demographic breakdown for the segment. This is the segment-level primary path: the breakdown is evaluated directly over the segment's own filter definition — no audience composition (set_logic or exclusions) is applied. For a composed view, use GET /v1/audiences/{id}/demographics on a referencing audience. Works for every subtype: matched, similarity, and propensity segments bridge through a materialized scratch id-set (a 1-membership composition) so the same catalog-driven breakdown logic can run against them. 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.