DespawnScheduler

Bounds the automatic despawn pipeline so a flood of despawning items can't storm the server. Items to relocate are enqueued; a once-per-tick drain starts at most performance.max-per-tick new DespawnProcesses and never exceeds performance.max-concurrent in flight. The queue is capped at performance.max-queue with a configurable drop policy.

This replaces the old model where every despawning item immediately spawned its own chunk-loading process with no upper bound.