SafeOutputTree.create() called fchmod(0o700) on a directory handle and
commit() fsynced a directory handle. Both are POSIX-only: Windows rejects
them with EPERM ("operation not permitted, fchmod"/"fsync"), so every
extract_artifact call failed with a generic io error and produced no output.
Skip the redundant directory chmod and directory fsync on Windows. mkdir
already applies the requested mode, and file contents are synced in write().
Add a Windows regression test and run it, plus the archive-safety test, in
the curated Windows CI lane so the behavior is covered.
morluto