Switch to Arena allocation for protobuf messages to reduce heap allocations. Pass ChunkDataRsp by value to enable move semantics. Cap thread pool size to hardware concurrency with max 4 threads.
Add `update_player_chunk()` method to `ClientPlayer` that triggers a chunk request when the player moves more than 2 chunks away from the last recorded chunk. Introduce an atomic `m_requesting_chunk` flag in `ClientWorld` to prevent concurrent requests. Rename `m_player_chunk_pos` to `m_last_chunk_pos` for clarity.
Remove monolithic World, Player, Chunk classes. Introduce ClientWorld, ServerWorld, ClientPlayer, and related networked components. Add Abseil dependency for logging and checks. Rename old files to pre_remove_* and update all includes and references accordingly.
