refactor(world): rework chunk state machine and player chunk tracking

Introduce ChunkState enum and ChunkEntity struct to manage chunk lifecycle. Store chunks as shared_ptr to avoid move operations during generation. Add clear_unused_chunks to remove chunks not referenced by any player. Implement deferred chunk request queue for safe processing after generation completes. Update player chunk set during required chunk computation. Improve thread safety with mutexes on chunk and player maps. Fix m_gening flag not reset after generation and add assertions for correctness. Change need_gen to require a player UUID, removing std::optional. Add chunk_size query method for debugging.

CMakeLists.txt

@@ -123,6 +123,9 @@ target_link_libraries(${PROJECT_NAME}
         protobuf::libprotobuf
         absl::log
         absl::check
+        absl::base
+        absl::strings
+        absl::flat_hash_map
         zstd::zstd
         $<$<PLATFORM_ID:Windows>:ws2_32>