A provider-tagged handle to a running (or attached) streaming command.
ref is the correlation key: every message the adapter delivers to the
owner process carries a map with this ref (see the message contract in
Managoat.Sandbox), and callers store it to match frames in
handle_info/2 heads. Consumers must match %{ref: ref} — never an
adapter's internal struct.
private is adapter-owned (the Sprites adapter keeps the SDK's command
struct there) and opaque to callers.