Make Peaceful spawn guard robust during world creation

[tastybento]

Follow-up to #15. The smoke test still crashed with the exact same stack trace because the released jar pre-dates that fix — and because the original guard was not robust enough for this code path.

Why #15 alone wasn't sufficient

The populator runs while the world is still being created, inside WorldCreator.createWorld() (Poseidon.getWorld:167 → createWorlds:140). At that point the world is not yet registered with Bukkit, so Bukkit.getWorld(worldInfo.getUID()) can return null, the peaceful flag evaluates to false, and monster spawns proceed — crashing the chunk system again:

Caused by: java.lang.IllegalStateException: Cannot spawn monster for org.bukkit.entity.Drowned in Peaceful difficulty
    at world.bentobox.poseidon.generator.PoseidonBlockPop.netherPop(PoseidonBlockPop.java:89)
Caused by: java.lang.RuntimeException: Chunk system crash propagated from unrecoverableChunkSystemFailure

(seen in https://github.com/BentoBoxWorld/bbox-test-harness/actions/runs/28332381857/job/83932707724)

Fix

• Fallback difficulty source: when the live world isn't registered yet, fall back to the addon's configured difficulty (Settings.getDifficulty()).
• Defensive spawn: wrap spawnEntity in a try/catch (IllegalStateException). This is the real guarantee — a failed spawn for any reason (difficulty restriction, detection mismatch) is caught locally and can never propagate into an unrecoverable chunk-system failure during world generation.

No further version bump needed — #15 already set the version to 1.1.1.

🤖 Generated with Claude Code