Add DXF export from the command line (--export-dxf)

qelectrotech --export-dxf <project.qet> <output_dir> [--show-terminals]
writes one DXF per folio, named <NN>_<title>.dxf like the PNG and SVG
exports. Discussion #1072.

The DXF code moves out of ExportDialog into DxfExport, unchanged except
that its options come from an ExportProperties argument instead of the
dialog. The dialog and the command line both call it, and the command
line uses the dialog's default options (the preferences' export
settings), so both write the same entities.

- Createdxf answers a file it cannot open with a message box and
  exit(0); the command line checks the file first and fails with a
  message and exit code 1 instead.
- A note is printed when pictures become outline boxes, as the dialog
  warns.
- Scripting: qet.exportDxf(outDir, showTerminals). MCP: format "dxf".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01G2d2Zi8BfrYRPX88zhaoFG

sources/cli_export.cpp

@@ -24,8 +24,11 @@
 #include "dataBase/projectdatabase.h"
 #include "diagram.h"
 #include "diagramcontext.h"
+#include "dxfexport.h"
+#include "exportproperties.h"
 #include "pdf_links.h"
 #include "qetgraphicsitem/conductor.h"
+#include "qetgraphicsitem/diagramimageitem.h"
 #include "qetgraphicsitem/element.h"
 #include "qetgraphicsitem/terminal.h"
 #include "qetproject.h"
@@ -69,6 +72,7 @@ const QHash<QString, QString> &exportFlags()
 		{"--export-pdf", "pdf"},
 		{"--export-png", "png"},
 		{"--export-svg", "svg"},
+		{"--export-dxf", "dxf"},
 		{"--export-cables", "cables"},
 		{"--export-wires", "wires"},
 		{"--export-bom", "bom"},
@@ -279,6 +283,57 @@ int exportImages(QETProject &project, const QString &format,
 	return 0;
 }
 
+/// One DXF file per diagram, written by the same code as the export dialog
+/// (DxfExport) with the dialog's default options -- the export settings of
+/// the preferences -- so both give the same file.
+int exportDxf(QETProject &project, const QString &out_dir, bool showTerminals)
+{
+	const QList<Diagram *> diagrams = project.diagrams();
+	if (diagrams.isEmpty()) {
+		err << "No diagrams to export.\n";
+		return 1;
+	}
+	QDir().mkpath(out_dir);
+
+	ExportProperties properties = ExportProperties::defaultExportProperties();
+	properties.format = QStringLiteral("DXF");
+	if (showTerminals)
+		properties.draw_terminals = true;
+
+	int index = 0;
+	bool has_images = false;
+	for (Diagram *diagram : diagrams) {
+		++index;
+		const QString path = QDir(out_dir).filePath(
+			diagramStem(diagram, index) + ".dxf");
+
+		// Createdxf answers a file it cannot open with a message box and
+		// exit(0), which headless means no message and a success code.
+		QFile probe(path);
+		if (!probe.open(QIODevice::WriteOnly)) {
+			err << "Cannot open '" << path << "' for writing.\n";
+			return 1;
+		}
+		probe.close();
+
+		const QSize size = DxfExport::folioSize(diagram, properties);
+		DxfExport::write(diagram, size.width(), size.height(), path, properties);
+
+		for (QGraphicsItem *item : diagram->items()) {
+			if (qgraphicsitem_cast<DiagramImageItem *>(item)) {
+				has_images = true;
+				break;
+			}
+		}
+		out << "  " << path << "\n";
+	}
+	if (has_images)
+		err << "Note: DXF has no pictures in this format; "
+			   "each picture is exported as an outline box.\n";
+	out << "Exported " << diagrams.size() << " diagram(s) -> " << out_dir << "\n";
+	return 0;
+}
+
 int exportCsv(QETProject &project, const QString &format, const QString &output)
 {
 	QString csv;
@@ -890,6 +945,8 @@ int run(const QStringList &args)
 	}
 	if (format == "pdf")
 		return exportPdf(project, output, showTerminals);
+	if (format == "dxf")
+		return exportDxf(project, output, showTerminals);
 	if (format == "cables" || format == "wires")
 		return exportCsv(project, format, output);
 	if (format == "bom")