WorkspaceLocks

The locks of a workspace's backends (design §4.33): a file mutation and a command alike lock the whole workspace, files/workspace/<id>, so a batch's edits and commands apply one after another in call order. A per-path key would not do: two paths can name one file through a link (/alias -> /real), and their edits must still be ordered. The key is under C3's FileBackend.LockRoot, as FileBackend.resolved requires of a resolved backend's locks.