[macmv]

Summary

• Moves Splotch, Splatter, WaterResources, Pool, RiverSide and MossBoulder from placer to chunk_placer, and updates all biome call sites to g.place_chunk(...).
• Splotch and WaterResources patches are seeded per source chunk, so they stay continuous across chunk borders. Pool, MossBoulder and RiverSide are kept inside one chunk.
• BiomeBuilder::generate now gives each chunk placer an rng seeded from the world seed and the placer index (independent of earlier placers). This changes the random patterns of the existing chunk placers too.

Behavior differences

• Round patches are now flat surface discs rather than 3D spheres.
• WaterResources now patches the floor of any water up to 10 deep (the old version only fired when the shifted column was lower than the anchor), so avg_per_chunk may need retuning.
• These run during base generation, before trees and other decoration, so e.g. podzol now exists before trees try to spawn.
• Per-placer names are gone, so placer_enabled! toggles no longer apply to these; they fall under BiomeChunkPlacers.

LavaLake, Pillar and Monument are left as regular placers.

Testing

cargo check --workspace and cargo test -p rgen-placer -p rgen-biome pass. Not yet verified visually in a generated world.