refactor(gameplay): move chunk generation phases into gen_chunk method

Consolidate multiple phase generation calls into a single gen_chunk() method on Chunk, which handles neighbor generation and ensures thread safety. Simplify World::gen_chunks_internal by using gen_chunk() instead of manual phase orchestration.

include/Cubed/gameplay/chunk.hpp

@@ -124,6 +124,8 @@ public:
     void need_upload();
 
     void set_chunk_block(int index, unsigned id);
+    // ensure thread safe!
+    void gen_chunk();
 
     ChunkPos chunk_pos() const;
     BiomeType biome() const;