Save a markdown document

Writes new bytes and bumps the SAME artifact to the next version — /documents/:id keeps its address — and puts one artifact entry on the timeline carrying summary. Send baseVersion — the version you edited — and a save that would overwrite a newer one answers 409 document_version_conflict instead. Without a summary the platform generates +N/−M lines from the diff. Authorization: the same gate as every session route — work_session:read (or comment to write) on the session, OR space:contribute on one of its linked spaces; a desk session only for its owner. Any denial is 404.