The DXF exporter was missing the $INSUNITS header variable, which tells
CAD software what physical units the drawing coordinates represent. Without
it, other programs have no reliable way to determine the coordinate scale.
Add $INSUNITS to the HEADER section alongside the existing $MEASUREMENT
variable. When exporting in millimeters, $INSUNITS is set to 4; when
exporting in inches, it is set to 1.
Fixes https://gitlab.com/kicad/code/kicad/-/issues/23181
Seth Hillbrand
c0a6a139cf