Save a .qetz without building the folios not touched (stage D4)

With QET_LAZY_FOLIOS=1, a project opened from a .qetz's index saves a
.qetz without building the folios it never built, when building them
would write the same: their parts are copied as they were and their rows
of the document's engineering data from the project.sqlite they were read
from (QetContainerDb::extract() with kept_from, QetContainer::split() with
the old entries, pictures included). Whether a folio may be kept follows
the rules of stage 5.5 (QETProject::toXml() with kept_parts): nothing
project-wide changed since opening; a folio whose own data changed is
built with its partners; after an edit, folios holding a table filled
from the database are built; the partners still waiting of every built
folio are built. Trusted only for a project opened from a format 3 index
written by this version whose folios are named by their uuid.

Format 3 names a folio by its uuid in the database (doc_meta folio_ids =
uuid) when every folio has one of its own, so that a kept folio keeps its
rows whatever its place; copies keep the rows' order (ORDER BY rowid).
A folio not built keeps the name of its part until it is built
(Diagram::deferredPart()).

Measured, the 136-folio f2099 opened from its .qetz and saved as a .qetz:
4.75 s with no folio built, 33 s with every folio built; its 136 folio
parts copied byte for byte; a kept save of a kept save is the same bytes.
industrial.qetz untouched: 28 of its 47 folios not built are built to save
(link chains, as in 5.5); after everyday edits on its busiest folio, 7
stay not built.

tst_lazyfolios: an untouched .qetz saved without building, read back,
holds the project; edits saved without building hold, read back, what the
same edits saved with every folio built hold (read back too: a wire
numbered from its potential is saved with no sequential numbers whichever
the way -- reading it back renumbers it, a separate issue); fails when a
kept folio's symbol information is not copied. Corpus, 342 projects: a
.qetz saved without building, read and saved as .qet = eager = master on
all 342. ctest
96/96 with and without QET_LAZY_FOLIOS=1.

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

sources/container/qetcontainer.cpp

@@ -22,6 +22,7 @@
 #include <QHash>
 #include <QMap>
 #include <QSet>
+#include <QRegularExpression>
 
 const char QetContainer::MimeType[] = "application/x-qelectrotech-project+zip";
 
@@ -228,7 +229,8 @@ void QetContainer::dropComputed(QDomElement diagram, bool terminal_lists)
 	set if @p project is not a project
 */
 QList<QetZip::Entry> QetContainer::split(const QDomDocument &project, const QString &writer,
-										 QString *error, const QByteArray &index)
+										 QString *error, const QByteArray &index,
+										 const QHash<QString, QByteArray> &kept_from)
 {
 	QDomDocument document = project.cloneNode(true).toDocument();
 	QDomElement root = document.documentElement();
@@ -238,7 +240,11 @@ QList<QetZip::Entry> QetContainer::split(const QDomDocument &project, const QStr
 	}
 
 		//The engineering data out of the document, into project.sqlite
-	const QByteArray database = QetContainerDb::extract(root, error, index);
+		//A folio written as it was read (<diagram qetz-kept-part="..."/>,
+		//stage D4) keeps its part and, through
+		//@p kept_from's project.sqlite, its rows
+	const QByteArray database = QetContainerDb::extract(
+				root, error, index, kept_from.value(QStringLiteral("project.sqlite")));
 	if (database.isEmpty())
 		return {};
 
@@ -247,11 +253,35 @@ QList<QetZip::Entry> QetContainer::split(const QDomDocument &project, const QStr
 
 		//Folios, each with its pictures taken out
 	int folio = 0;
-	QMap<QString, QByteArray> pictures;   //name -> png, once each
 		//Nothing QElectroTech computes is kept (format 3)
 	for (QDomElement diagram : childElements(root, QStringLiteral("diagram")))
 		dropComputed(diagram, !index.isEmpty());
 
+		//The parts of the folios kept as they were, and their pictures
+	QMap<QString, QByteArray> pictures;   //name -> png, once each
+	for (QDomElement diagram : childElements(root, QStringLiteral("diagram")))
+	{
+		const QString kept = diagram.attribute(QStringLiteral("qetz-kept-part"));
+		if (kept.isEmpty()) continue;
+		if (!kept_from.contains(kept)) {
+			setError(error, QStringLiteral("the part \"%1\" of a folio kept as it was is missing").arg(kept));
+			return {};
+		}
+		taken.insert(kept);
+		const QByteArray part = kept_from.value(kept);
+		static const QRegularExpression image_file(QStringLiteral("%1=\"(images/[^\"]+)\"").arg(ImageFileAttribute));
+		for (const auto &match : image_file.globalMatch(QString::fromUtf8(part))) {
+			const QString name = match.captured(1);
+			if (!kept_from.contains(name)) {
+				setError(error, QStringLiteral("the picture \"%1\" of a folio kept as it was is missing").arg(name));
+				return {};
+			}
+			pictures.insert(name, kept_from.value(name));
+		}
+		parts.append(QetZip::Entry{kept, part, true});
+		root.replaceChild(placeholder(document, kept), diagram);
+	}
+
 	for (QDomElement diagram : childElements(root, QStringLiteral("diagram")))
 	{
 		++folio;