Add operations (erode, fill, flood, pencil), filter system, layer system with exporters, and WorldPainter .world import
- New operations: ErodeOperation (thermal erosion), FillOperation (flood-fill), FloodOperation (hard replace), PencilOperation (single-point)
- Filter system: Filter trait with TerrainFilter, HeightFilter, SlopeFilter, CompositeFilter, InvertFilter, FilteredOperation wrapper
- Layer system: Layer trait + 6 implementations (Caves, River, Frost, Trees, Biome, Resources) + layer data helpers on Tile
- Layer exporters: LayerExport trait + 6 implementations that modify blocks during export (caves carve air, rivers create water channels, frost adds snow/ice, trees place logs, resources add ores, biomes set biome palette)
- WorldPainter .world import: Java serialization stream parser with full TC_* token support, tile extraction, format auto-detection
loki5512344
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