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/ui/terminalnumberingdialog.cpp

@@ -15,7 +15,7 @@
 
 namespace {
 	//The folios holding a terminal, built: a folio not built yet
-	//(QET_LAZY_FOLIOS) holding none is not built for this
+	//(lazy folios) holding none is not built for this
 QList<Diagram *> terminalFolios(QETProject *project)
 {
 	project->buildFoliosHolding(ElementData::Terminal, false);