The service-generated upload id (authoritative; scoped to the napplet).
The napplet window that requested the upload.
Push a status update (progress, or a transition to complete/failed). The
service stamps uploadId and updatedAt before forwarding to the napplet
as upload.status.changed, and records it as the latest tracked status.
Context handed to an Uploader for a single upload. Carries the service-owned
uploadIdand a sink for streaming progress / state changes.