Include host_counts on every artifact returned: how many hosts share it, by each match hash. Opt-in because it is the one part of the search whose cost grows with how widespread the artifacts are, and capped per hash so a widespread artifact reports a floor rather than an exact number (see ArtifactHostCounts). Only available for pages of up to 200 artifacts; a larger size with this set is rejected. The artifact detail endpoint always reports these counts, and without a cap.