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/diagramview.cpp

@@ -1649,7 +1649,7 @@ void DiagramView::editDiagramProperties()
 	@brief DiagramView::buildFolio
 	Build the items of the folio this view shows, and those of the folios
 	its cross-references need, if the project was opened without them
-	(QET_LAZY_FOLIOS); then fit the view to them.
+	(lazy folios); then fit the view to them.
 */
 void DiagramView::buildFolio()
 {