Show a folio without building its partners' folios (stage D5b)

Showing a folio built it and every folio holding a partner of its
symbols. Now a partner is built alone, as a symbol, into its folio's
scene (Diagram::buildSymbols()), and reused when that folio is built.

- A linked folio report built alone had no wires when it was linked,
  so its texts showing a wire's properties stayed empty: after any
  build, linked reports on the built folios and their partners look at
  their wires again, as opening does. Unlinked reports keep their texts,
  as opening leaves them.
- A report whose partner was built alone builds the partner's folio
  when its potential is followed.
- tableChainFolios() parsed every unbuilt folio's file. The index now
  lists the drawn tables and the table each continues (folio_table),
  and a .qetz opened from it uses that list. An index without it falls
  back to the folio files.
- A .qetz's project.sqlite is opened once per project
  (QetContainerDb::FolioReader), not once per folio built.

nuri (136 folios), opened fresh, one folio shown: median 1104 -> 184 ms,
each show builds one folio; industrial 234 -> 87 ms.

tst_lazyfolios: a folio whose table continues another is shown first
from a .qetz (fails when the index's chains are ignored). ctest 96/96
eager and lazy; 342-project corpus: master, lazy, .qetz lazy and .qetz
kept all the same.

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

sources/container/qetcontainerdb.cpp

@@ -929,6 +929,47 @@ bool QetContainerDb::restoreFolio(QDomElement diagram, const QByteArray &databas
 	return openDatabase(db, database, error) && restoreDiagram(db, diagram, error);
 }
 
+struct QetContainerDb::FolioReader::Private
+{
+	QByteArray database;
+	Database db;
+	bool opened = false;
+	QString open_error;
+};
+
+/**
+	@brief QetContainerDb::FolioReader::FolioReader
+	Reads folios' engineering data from @p database, opened the first time
+	one is read and kept open: a folio built after the project was opened
+	from a .qetz no longer copies the whole database (stage D5)
+*/
+QetContainerDb::FolioReader::FolioReader(const QByteArray &database) :
+	d(std::make_unique<Private>())
+{
+	d->database = database;
+}
+
+QetContainerDb::FolioReader::~FolioReader() = default;
+
+/**
+	@brief QetContainerDb::FolioReader::restore
+	restoreFolio() for @p diagram
+*/
+bool QetContainerDb::FolioReader::restore(QDomElement diagram, QString *error)
+{
+	if (!d->opened) {
+		d->opened = true;
+		if (!openDatabase(d->db, d->database, &d->open_error) && d->open_error.isEmpty())
+			d->open_error = QStringLiteral("the project database cannot be read");
+		d->database = QByteArray();
+	}
+	if (!d->open_error.isEmpty()) {
+		setError(error, d->open_error);
+		return false;
+	}
+	return restoreDiagram(d->db, diagram, error);
+}
+
 /**
 	@brief QetContainerDb::foliosByUuid
 	@return true if @p database, a format 3 project.sqlite, names its