Add BiomeProvider structure search fast path to CustomWorldChunkManager

[diytechy]

Adds getStructurePlacementBiome(WorldInfo, int, int) to the Bukkit BiomeProvider API as a default method returning Optional.empty(). Generators may override this to return a cheap 2D biome approximation for structure placement eligibility checks (e.g. land vs ocean for stronghold ring position computation), bypassing expensive 3D pipeline evaluation for standard terrain generation.

CustomWorldChunkManager overrides findBiomeHorizontal, setting a ThreadLocal flag while the search runs so getNoiseBiome can call getStructurePlacementBiome() instead of the full getBiome() path. This avoids the extensive pipeline calculation cost for biome placement in more complex terrain generators like Terra.

Falls back silently to getBiome() if the provider returns empty, CraftBiome.bukkitToMinecraftHolder returns null, or the generator does not implement getStructurePlacementBiome.

Includes one-shot diagnostic logging to confirm the fast path is active and being used during stronghold searches.

[kennytv]

Do you need the debug logging or was it more for initial confirmation that it works? otherwise I'd just remove it since it ends up being more atomic logging code than implementation