Export: the prep, not the exporter

Export failures are almost never exporter bugs. They are prep that did not happen, repeated once per asset: an unapplied scale, an origin at the bounding-box centre, a material called Material.003, an axis convention that differs from the one the last batch used. These pieces are the checklist and the reasoning behind each item, plus how to audit a scene against a convention instead of checking objects one at a time.