The #26 fix makes has_staged_update require the .download-complete
marker, so the state self-heal treats a marker-less staging dir as a
partial download and clears update_in_progress. The roundtrip test
staged a binary file but not the marker, so it began failing. Write
the marker to simulate a *complete* staged update.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>