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/dataBase/projectdatabase.h

@@ -160,7 +160,7 @@ class projectDataBase : public QObject
 			//they are, say how many of those values building agreed with.
 		void prefillFromDocument(const QDomDocument &document);
 		void endPrefill(bool keep_unplaced = false);
-			//A folio of a project opened with QET_LAZY_FOLIOS is being built:
+			//A folio of a project opened with lazy folios is being built:
 			//its rows came from the document already, so none are written
 			//and the database is not marked changed
 		void setBuildingFolio(bool building);
@@ -357,7 +357,7 @@ class projectDataBase : public QObject
 			//The folios whose rows a rebuild keeps (keepUnbuiltFolioRows())
 		QSet<Diagram *> m_kept_folios;
 			//The tables' rows were filled from the document: those of a
-			//folio not built yet are right (QET_LAZY_FOLIOS)
+			//folio not built yet are right (lazy folios)
 		bool m_rows_from_document = false;
 		void keepUnbuiltFolioRows();
 		void restoreKeptRows(const QString &table, Diagram *folio, const QString &where);