Save a new title, author or revision without building the folio (stage D5)

A folio not built whose own data changed was built to save, with the
folios its links reach. When what changed is only title block fields
nothing on a folio is computed from -- its title, author, date,
revision index, version, file name, template; a formula reads the
folio's label, plant, location, extra fields and grid -- its items as
read are what building it would write: it is saved from them with its
own data as it is now (Diagram::deferredXmlWithHeader()), .qet and
.qetz alike, with the information of its symbols changed through the
project database (or built, when one of those needs it). Any other
change still builds it.

136-folio .qetz, revision index and author set on every folio by a
script: 0.8 s, saved in 6.8 s, no folio built; with every folio built,
the same file. tst_lazyfolios: a new revision on every folio of a .qet
and of a .qetz saved as when built, building only the folios holding a
table (fails, 47 built, when every change counts); a folio whose label
changed is built (fails when none counts). ctest 96/96 eager and lazy;
342 corpus projects save the same in all four arms and export the same.

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

sources/diagram.cpp

@@ -830,6 +830,57 @@ QList<Element *> Diagram::buildSymbols(const QSet<QUuid> &uuids)
 	return built;
 }
 
+namespace {
+	//The title block fields nothing on a folio is computed from: a formula
+	//reads the folio's label, plant, location, extra fields and grid
+	//(autonum::FormulaContext), none of these
+const QStringList ShownOnlyFields = {
+	QStringLiteral("title"), QStringLiteral("author"), QStringLiteral("date"),
+	QStringLiteral("indexrev"), QStringLiteral("version"), QStringLiteral("filename"),
+	QStringLiteral("titleblocktemplate"), QStringLiteral("titleblocktemplateCollection")};
+
+QString headerWithoutShownOnly(const QString &header)
+{
+	QDomDocument document;
+	document.setContent(header);
+	QDomElement root = document.documentElement();
+	for (const QString &field : ShownOnlyFields) root.removeAttribute(field);
+	return document.toString();
+}
+}
+
+/**
+	@brief Diagram::headerChangeShowsOnly
+	@return true if what changed in this folio's own data since it was read
+	(headerChanged()) is only title block fields nothing on the folio is
+	computed from (its title, author, date, revision, file name, template):
+	its items, as read, are what building it would write
+*/
+bool Diagram::headerChangeShowsOnly()
+{
+	return headerWithoutShownOnly(headerXml()) == headerWithoutShownOnly(m_opened_header);
+}
+
+/**
+	@brief Diagram::deferredXmlWithHeader
+	@return this folio, not built, as it saves: its own data as it is now
+	(writeHeader()) and its items as read (deferredXml()), in @p document --
+	when its items are what building it would write (headerChangeShowsOnly())
+*/
+QDomElement Diagram::deferredXmlWithHeader(QDomDocument &document)
+{
+	QDomElement folio = document.createElement(QStringLiteral("diagram"));
+	writeHeader(document, folio);
+	QSet<QString> header_children;
+	for (QDomElement child = folio.firstChildElement() ; !child.isNull() ; child = child.nextSiblingElement())
+		header_children.insert(child.tagName());
+	const QDomElement items = deferredXml();
+	for (QDomElement child = items.firstChildElement() ; !child.isNull() ; child = child.nextSiblingElement())
+		if (!header_children.contains(child.tagName()))
+			folio.appendChild(document.importNode(child, true));
+	return folio;
+}
+
 /**
 	@brief Diagram::deferredXml
 	@return the XML of this folio's items while they are not built: kept