Per-client round-robin over a shrinking job list needs a last-served tiebreak

Ranking waiting jobs by 'how many older jobs does this client still have queued' looks like round-robin but isn't: as one client's jobs finish, its oldest remaining job is rank 0 again, ties go to the oldest sequence number, and a client with 50 queued jobs still starves a newcomer whose single job also has rank 0 (stub: 5.2 s wait before, 0.12 s after the fix). Add a second key: a counter of when each client last had a job picked, lowest first. Key order that worked: [priority class, per-client rank, client last-picked counter, remaining work, sequence]. Record the pick for every client attached to the chosen job (shared jobs can serve several). Keep the old behaviour with one client attached, so single-user paths are unchanged.