Snapshot a STOPPED node's parked volume to durable storage

Snapshot a node's parked volume to durable storage (M9c, spec 'Volume snapshots'): a copy of the disk you keep - protect an hour of downloaded-and-converted state against anything that happens to the machine. The node must be stopped (stop_node parks the disk intact; the snapshot is then a clean point-in-time copy) - a node still "stopping" is saving its last writes and refuses until get_node reads stopped - and it must STAY stopped while the upload runs: starting the node cancels an in-flight snapshot (the copy can't stay clean under a live disk) - the row resolves failed, and you re-snapshot after the next stop. Returns {snapshot_id, status: "uploading"} immediately - never a blocked call; poll list_snapshots until it's ready. Later work on the node is NOT in the snapshot, and /scratch is never in it (a snapshot is the kept disk; scratch is destroyed at stop). Snapshots are bounded per org (count and total bytes) and never expired on a timer. Snapshots ride the volume's encryption: the object store never holds plaintext. To save progress from work while it RUNS, use a job instead (submit_job): jobs checkpoint without stopping.