Add $INSUNITS header variable to DXF export

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

common/plotters/DXF_plotter.cpp

@@ -701,12 +701,14 @@ void DXF_PLOTTER::SetUnits( DXF_UNITS aUnit )
     case DXF_UNITS::MM:
         m_unitScalingFactor = 0.00254;
         m_measurementDirective = 1;
+        m_insUnits = 4;
         break;
 
     case DXF_UNITS::INCH:
     default:
         m_unitScalingFactor = 0.0001;
         m_measurementDirective = 0;
+        m_insUnits = 1;
     }
 }
 
@@ -779,6 +781,10 @@ bool DXF_PLOTTER::StartPlot( const wxString& aPageNumber )
                 "$MEASUREMENT\n"
                 "  70\n"
                 "{}\n"
+                "  9\n"
+                "$INSUNITS\n"
+                "  70\n"
+                "{}\n"
                 "  0\n"
                 "ENDSEC\n"
                 "  0\n"
@@ -873,7 +879,7 @@ bool DXF_PLOTTER::StartPlot( const wxString& aPageNumber )
                 "-0.2\n"
                 "  0\n"
                 "ENDTAB\n",
-                GetMeasurementDirective() );
+                GetMeasurementDirective(), GetInsUnits() );
 
     // Text styles table
     // Defines 4 text styles, one for each bold/italic combination