DespawnScheduler

Bounds the automatic despawn pipeline so a flood of despawning items can't storm the server, and shares that bounded budget fairly between players.

Two independent layers:

• Server budget (performance:) — at most max-per-tick new relocations start each tick and never more than max-concurrent run at once. This is what stops the server from being overwhelmed.

• Per-user budget (throttle:) — how much of that server budget any one player may take, via io.fairyfox.papermc.despawneditems.throttle.ThrottleManager. This is what stops one player from starving everyone else.

The void.chance roll happens here too, at enqueue time rather than per relocation attempt: an item's fate should be decided once, not re-rolled at every candidate location.