Export the master-side cross-reference table to DXF

Follow-up to #740, which fixed the slave-side "(n-Xn)" cross-reference
label. The master-side item - the small table/cross drawn next to a
report or master element, listing where each of its slaves is used -
was still missing from DXF export. Measured against examples/
industrial.qet with the PDF export as an oracle (renders the whole
scene, so it shows what should be there):

                          before  after   PDF
  slave xrefs  "(n-Xn)"       41     41    41   (already fixed, #740)
  folio/position strings     358    403   403

DXF now matches the PDF exactly.

## Why this needed a different approach than #740

The slave label is a plain QGraphicsTextItem - one string, trivial to
walk and re-emit as a single DXF TEXT entity, which is what #740 did.

The master-side item (CrossRefItem) is not: it paints itself with
~600 lines of hand-written QPainter calls across three modes
(drawAsCross/drawAsContacts/drawAsPlcTable), including a header
table, contact symbols, and rules. Hand-porting that logic to emit
DXF primitives directly would mean maintaining two divergent
implementations of the same drawing that have to be kept in sync by
hand forever.

## Approach: a QPaintEngine that intercepts CrossRefItem's own paint()

DxfPaintEngine/DxfPaintDevice (sources/dxfpaintdevice.{h,cpp}) is a
QPaintEngine/QPaintDevice pair - the same mechanism QPrinter and
QSvgGenerator use to redirect QPainter output elsewhere. Constructing
a QPainter on a DxfPaintDevice and calling item->paint() on it produces
DXF entities instead of pixels, using the exact same drawing code that
already renders correctly on screen. CrossRefItem::paint() is
unmodified.

Scope is deliberately narrow - only the QPainter calls CrossRefItem's
paint() is observed to make: drawLines -> LINE, drawRects/drawPath's
fill case -> outline-only LWPOLYLINE (no HATCH support in v1 - DXF's
fill primitive is a separate, more involved entity type; documented as
a known limitation rather than attempted here), drawEllipse -> CIRCLE
or a flattened polygon for rotated ellipses, drawPath's arc case (from
drawArc/drawPie) -> chord-flattened LINE segments, drawPolygon ->
LWPOLYLINE, drawTextItem -> TEXT. drawPixmap is intentionally
unimplemented (qWarning + skip) since CrossRefItem never calls it -
this is not a general-purpose DXF paint engine, and isn't meant to be
in this PR.

CrossRefItem::paint() is protected, per the normal QGraphicsItem
contract - added a small paintForExport() wrapper rather than making
paint() itself public, or reaching around access control.

## Explicitly out of scope

QetShapeItem::toDXF() and QetGraphicsTableItem::toDXF() (both already
implemented and working) are untouched. Rewriting working exporters
onto this engine to prove an architectural point would be a large,
unrelated diff with no user-visible benefit - if that consolidation is
wanted later, it's a separate proposal once this engine has shipped
and proven out on the one item that currently has no DXF export at
all.

## Testing

Built clean on Qt5/Linux. Verified via the GUI export dialog
(Fichier > Exporter > DXF) against examples/industrial.qet, 50 folios:
export completes without error or crash, all 50 .dxf files are
structurally well-formed (balanced SECTION/ENDSEC, single EOF each),
and grepping the folio-position pattern gives the before/after/PDF
numbers above. Spot-checked several real label strings (e.g. "18-B18",
"20-A2") present as TEXT entity values in the output, not just an
artifact of the count matching.

sources/exportdialog.cpp

@@ -22,8 +22,10 @@
 #include "exportpropertieswidget.h"
 #include "factory/elementpicturefactory.h"
 #include "qetgraphicsitem/ViewItem/qetgraphicstableitem.h"
+#include "dxfpaintdevice.h"
 #include "qetgraphicsitem/conductor.h"
 #include "qetgraphicsitem/conductortextitem.h"
+#include "qetgraphicsitem/crossrefitem.h"
 #include "qetgraphicsitem/diagramimageitem.h"
 #include "qetgraphicsitem/diagramtextitem.h"
 #include "qetgraphicsitem/dynamicelementtextitem.h"
@@ -467,6 +469,12 @@ void ExportDialog::generateDxf(
 		//as plain QGraphicsTextItem children, so neither cast below picks them
 		//up and they were missing from the DXF entirely.
 	QList<QGraphicsTextItem *> list_xref_texts;
+		//Master-side cross-reference item (the table/cross drawn next to a
+		//report/master element). It paints itself with hand-written
+		//QPainter code across three modes (drawAsCross/drawAsContacts/
+		//drawAsPlcTable), so instead of hand-porting each one it's replayed
+		//through DxfPaintEngine, which reuses paint() unmodified.
+	QList<CrossRefItem *> list_master_xrefs;
 	QList<QLineF *> list_lines;
 	QList<QRectF *> list_rectangles;
 	//QList<QRectF *> list_ellipses;
@@ -493,6 +501,8 @@ void ExportDialog::generateDxf(
 			}
 		} else if (QetGraphicsTableItem *gti = qgraphicsitem_cast<QetGraphicsTableItem *>(qgi)) {
 			list_tables << gti;
+		} else if (CrossRefItem *xref = qgraphicsitem_cast<CrossRefItem *>(qgi)) {
+			list_master_xrefs << xref;
 		}
 	}
 
@@ -722,6 +732,18 @@ void ExportDialog::generateDxf(
 		}
 	}
 
+	//Draw the master-side cross-reference items (table/cross), replaying
+	//their existing paint() unmodified through DxfPaintEngine instead of
+	//hand-porting drawAsCross()/drawAsContacts()/drawAsPlcTable().
+	for (CrossRefItem *xref : std::as_const(list_master_xrefs))
+	{
+		DxfPaintDevice dxf_device(file_path);
+		QPainter painter(&dxf_device);
+		painter.setWorldTransform(xref->sceneTransform());
+		xref->paintForExport(&painter);
+		painter.end();
+	}
+
 	Createdxf::dxfEnd(file_path);
 
 	saveReloadDiagramParameters(diagram, false);