Cancel blocked natural spawns before the entity is created

[RasmusKD]

The mob-spawning and deny-spawn checks currently run in CreatureSpawnEvent, which fires at the very end of the spawn pipeline. By that point the server has picked a spawn position, run the placement checks, constructed the mob and run finalizeSpawn, and the cancelled mob is thrown away. Because a cancelled spawn never counts toward the mob cap, the natural spawner keeps retrying the same area at full rate for as long as those conditions hold, so regions that deny mob spawning become permanent spawn attempt hotspots that pay entity construction over and over for nothing. On a production server (61 players) we profiled recently, natural spawn machinery accounted for roughly a quarter of the main thread, and areas denying spawns contribute to that without anything visible happening.

This PR adds a listener for Paper's PreCreatureSpawnEvent that applies the same natural spawn checks before the entity exists: activity halt, block-creature-spawn, the mob-spawning and deny-spawn region flags, and block-ground-slimes. Cancelling at the pre event stage also makes the server end the remaining spawn attempts for that chunk in the current cycle, so the retry pressure disappears as well.

Measured on a flat test world with a region denying mob-spawning across the whole spawn range and the mob cap kept empty: the attempt rate around a single player collapsed from roughly 75000 attempts per second reaching the pre spawn stage to roughly 60 per second, with no entities constructed at all.

Only NATURAL spawns are handled here. Every other spawn reason keeps going through the existing CreatureSpawnEvent checks unchanged, so there is no behavior change for spawners, breeding, plugins spawning entities and so on. The listener is registered only when the Paper event class is present, using the same PaperLib gate style as elsewhere in the plugin.

One thing worth noting for review: plugins that watch CreatureSpawnEvent will no longer see the natural spawns WorldGuard denies, since those never reach entity creation anymore. Anything that wants to observe or override them can use PreCreatureSpawnEvent.

Edit, measurement details after a follow-up test:

The attempt counter counts PreCreatureSpawnEvent dispatches, so positions that already passed the placement checks and are one step from entity construction. It was a LOWEST priority listener on an otherwise idle flat world with a single stationary player, difficulty hard, permanent night.

Scenario	Attempts reaching pre spawn stage	Entities constructed
No denial, mob cap kept empty	~75,000/s	normal spawns
Denial cancelled at CreatureSpawnEvent (current behavior, simulated with a cancel-all listener)	~97,000/s sustained	~4,500/s constructed and discarded, sustained for the whole measurement window
Denial cancelled at PreCreatureSpawnEvent (this PR)	~60/s	none

So the current late cancel does not just waste the construction work, it sustains the maximum spawn attempt pressure under the tested conditions, while the pre event cancel lets the spawn cycle stop early and the whole workload collapses.

[me4502]

So from what I can tell here, this line onwards is basically identical to the current CreatureSpawnEvent listener. Would it be possible to please extract this into its own method, similar to how the pig zap event handler does it? As these don't have a shared ancestor, you could just pass the event in as a Cancellable to allow cancelling it, and the necessary data in as parameters.

[RasmusKD]

done, extracted it into handleCreatureSpawn. the entity-specific checks stayed in onCreatureSpawn since there's no entity yet at pre-spawn time. re-tested on a live server, behaviour is identical.

[me4502]

Thanks for making these other fixes. Just a final one IMO, if you'd be able to please make cfg & wcfg passed in here too. Both are already available in the CreatureSpawnEvent handler, and the Pre one already pulls in cfg, so it'd remove the need for a second lookup (performance-wise)

[RasmusKD]

done, both passed in now