Save Search as a static List

Snapshots the people currently matching the given Search into a brand-new static List (returned as its wrapper Search). The membership is frozen at snapshot time — later changes to the source search or underlying data do not affect the list.

The snapshot always runs in a background worker (Core::CreateListFromSearchWorker) backed by a Task; the response includes the task_id the caller can poll. On completion the task deep-links to the new wrapper Search.

Gated behind the show_list_searches client setting — lists are only useful as audiences when that setting is on.