smolvm machine exec requires stdout to be a pipe, not a regular
file descriptor. Passing stdout=file caused smolvm to return
non-zero with no stderr (the error was silently swallowed or went
to the regular-file fd instead of reaching us).
Switch _snapshot_running_vm to a new _exec_tar_to_file helper that
uses Popen with stdout=PIPE and streams the tar to disk via
shutil.copyfileobj. A background thread drains stderr concurrently
to prevent deadlock when the stderr pipe buffer fills while we are
writing stdout data.
didericis-claude
f2775101a0