Introduce DataPackManager to generate/install a datapack into the target world folder (biome overrides, simplified noise_settings) and call install() during onLoad after the world folder is prepared. Add SurfaceBlockPopulator to apply surface/sub-surface block overrides (for TerraformGenerator worlds) and register it in onEnable via a WorldInitEvent listener for the configured target world. Refactor WorldManager to extract deletion logic into deleteWorld() and delete existing world/_nether/_the_end before unpacking the selected map. Also update imports and wiring to support these changes.
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