Download a template as a file

The layout, what it needs and where it came from — no dataset ids and no tags. Filing is a decision about the gallery rather than part of the template, so it stays on the row: a file exported here imports against any build, and a vocabulary rename touches no artifact. Layout-only unless data=1, which packages a capped copy of the rows the template opens on through the SAME writer the editor's door uses — two doors emitting their own bundles is how the caps and the row report drift apart.