The not(unix) arm returned Err and both preflight() and prepare()
propagate it, so the catalog never bootstrapped on Windows and the Local
Library died at preflight.
GetDiskFreeSpaceExW on the nearest existing ancestor, since the catalog
root may not exist on first run; LockFileEx with FAIL_IMMEDIATELY, mapping
ERROR_LOCK_VIOLATION to BootstrapBusy like the flock arm and any other
error to Io. Nothing reads this lock file's contents, so the whole-file
range is safe here.
cc
vicrodh
57f900bec4