Describe the bug
Monsters (like slimes) aren't in the Added list when they are added to the game state. When I enter a new mine level, I would expect to see a list of the added monsters in my callback.

To Reproduce
Log the Name of each NPC that gets Added in World.NpcListChanged. Go to the mine, and see that it doesn't get called for new monsters.

NpcListChanged is raised when monsters are added/removed in an existing location, but it's not raised for monsters that were already in the location when it was created. That's by design, since the event tracks changes to the NPC list for each location (and nothing changed if they were already there). I can document that more explicitly, but feedback is always welcome if that implementation is confusing.

Clarified in the event documentation.