Dynamic Trees Physics

Fixes a crash that could take the server down and lose the save. Chopping the last block out of a tree leaves an empty sub-level, and Sable asks every sub-level for its velocity while saving - an empty one answers with an exception, thrown from inside the world save. Finished trees are now retired at the head of Sable's own save instead of on a two-second tick, which closed a window an autosave landed in surprisingly often (opening the pause menu triggers one, and that is usually the very next keypress after felling a tree).