Add new packet S2C_ClearAllChunks (ID 3005) to notify clients when the server rebuilds its world. Clients respond by clearing their chunk cache and requesting fresh chunks. Shift UPDATE_TIME packet ID to 3006 to accommodate.
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.
