refactor(client_player): return ChunkPosSet by value in non-const getter

Copy the internal set under the mutex lock to avoid exposing a mutable reference that could be used unsafely after the lock is released. This fixes a potential data race when the caller holds the returned reference beyond the critical section. Also update the caller in `client_world` to remove the now-unnecessary `std::move`.

include/Cubed/gameplay/client_player.hpp

@@ -25,7 +25,7 @@ public:
 
     void update_chunk_set(const ChunkPosSet& set);
     const ChunkPosSet& get_chunk_pos_set() const;
-    ChunkPosSet& get_chunk_pos_set();
+    ChunkPosSet get_chunk_pos_set();
 
     static AABB get_aabb(const glm::vec3& pos);
     const glm::vec3& get_front() const;