Search sessions

A search session remembers which papers it has returned. Reusing its session_id suppresses those papers in later searches, including papers recorded through get_papers.

Sessions are optional. Calls without a session_id are stateless, and different sessions do not share state.

The session ID is a private capability. The server stores only its hash and cannot list or recover raw IDs. Sessions expire after inactivity and have bounded rounds and result counts.