Build folios only when needed, by default (stage D6)

Every project now opens without building its folios; a folio is built
when it is shown or something needs its items (QETProject::lazyFolios()).
QET_LAZY_FOLIOS is gone; QET_EAGER_FOLIOS=1 builds every folio on
opening, as before, for comparisons and troubleshooting. Comments that
named the old variable now say "lazy folios"; the tests switch modes
with the new one.

The first save of a .qet as .qetz still builds every folio: a .qet does
not say which program wrote it, so its folios are written again rather
than copied. A .qetz opens and saves without building.

ctest 96/96 by default and with QET_EAGER_FOLIOS=1; 342 corpus projects
save the same in all four arms and export the same. In the GUI
(qet-gui-dialog): a 136-folio .qetz opens with no folio built, shows
folios 1 to 4, saves building none, the save = the original but the time
spent in it; a .qet opens lazily, saves as a .qetz beside it after the
notice, the .qet unchanged.

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

sources/qetgraphicsitem/ViewItem/projectdbmodel.cpp

@@ -323,7 +323,7 @@ void ProjectDBModel::setIdentifier(const QString &identifier) {
 
 /**
 	@brief ProjectDBModel::folioBuilt
-	The folio of a table made while it was being built (QET_LAZY_FOLIOS)
+	The folio of a table made while it was being built (lazy folios)
 	is built: read the database, as opening does once it has filled it.
 	Nothing for any other model.
 */
@@ -391,7 +391,7 @@ void ProjectDBModel::fillValue()
 {
 	m_record.clear();
 		//Opening builds a table before the database is filled, and fills
-		//it after: a table on a folio built late (QET_LAZY_FOLIOS) does
+		//it after: a table on a folio built late (lazy folios) does
 		//the same, so that it is laid out as it would have been on
 		//opening -- empty until its folio is built (folioBuilt())
 	if (m_fill_when_built) {