Sync the folder after saving, and save from the command line through a temporary file

QSaveFile writes a new file, forces it to disk and renames it over the
old one, but does not force the folder to disk. On ext4 or XFS a power
cut soon after a save can then lose the rename: the previous file comes
back, or none. QET::syncFolderOf() now syncs the folder after every
project, symbol and title block save (QET::writeXmlText()) and every
.qetz save (QetZip::write()). Nothing to do on Windows.

--resave and --set-titleblock wrote a .qet straight over the output file
(QFile, truncate, write): a run stopped part-way left half a project.
They now write through QET::writeXmlText() like the GUI's save. On
Windows their .qet now has LF line ends, as the GUI writes.

Checked with strace on --resave: fdatasync of the new file, linkat into
place, fsync of the folder. The .qet written is byte-identical to before.
ctest 106/106.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

sources/utils/syncfolder.h

@@ -0,0 +1,54 @@
+/*
+	Copyright 2006-2026 The QElectroTech Team
+	This file is part of QElectroTech.
+
+	QElectroTech is free software: you can redistribute it and/or modify
+	it under the terms of the GNU General Public License as published by
+	the Free Software Foundation, either version 2 of the License, or
+	(at your option) any later version.
+
+	QElectroTech is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License
+	along with QElectroTech. If not, see <http://www.gnu.org/licenses/>.
+*/
+#ifndef SYNCFOLDER_H
+#define SYNCFOLDER_H
+
+#include <QFile>
+#include <QFileInfo>
+#include <QString>
+
+#ifndef Q_OS_WIN
+#include <fcntl.h>
+#include <unistd.h>
+#endif
+
+namespace QET {
+	/**
+		Force to disk the folder holding @p file_path, once
+		QSaveFile::commit() renamed a new file into it. QSaveFile syncs the
+		file it wrote but not the folder, and on some file systems (ext4,
+		XFS) a power cut soon after can then lose the rename: the previous
+		file comes back, or none. Does nothing on Windows, where a folder
+		cannot be synced this way.
+	*/
+	inline void syncFolderOf(const QString &file_path)
+	{
+#ifndef Q_OS_WIN
+		const QByteArray folder = QFile::encodeName(QFileInfo(file_path).absolutePath());
+		const int fd = ::open(folder.constData(), O_RDONLY | O_DIRECTORY | O_CLOEXEC);
+		if (fd >= 0) {
+			::fsync(fd);
+			::close(fd);
+		}
+#else
+		Q_UNUSED(file_path)
+#endif
+	}
+}
+
+#endif // SYNCFOLDER_H