Update an audience

Normalized, platform-agnostic ad-targeting spec. Every field is optional, an empty object targets the platform's default broadest audience. Field names are camelCase and identical across POST /v1/ads/create (the targeting object), POST /v1/ads/targeting/reach-estimate, and saved_targeting audiences, so a spec resolved once can be reused verbatim.

Entity ids (regions[].key, cities[].key, zips[].key, metros[].key, interests[].id, behaviors[].id) are the platform's opaque identifiers resolved via GET /v1/ads/targeting/search. A spec is therefore meaningful only for the platform it was built against, except the portable fields (countries, ageMin/ageMax, gender, incomeTier, languages) which carry across platforms. Fields a platform cannot honour are rejected at create time with INVALID_FIELD_VALUE naming the offending field (not silently dropped).