.qetz: no columns for where a block sat; open a later format read-only

The database held where each moved block sat among its parent's
children (props_at, info_at, links_at), though reading only asked
whether there was one: the marker the folio file keeps says that, and
where it goes back. The columns are gone; format 2 files leave the same
markers and still open.

A .qetz of a later format whose min-reader allows this version was
still refused by the database's own format check. It opens now,
read-only, with a message: saving it here could lose what this version
does not know. Save As writes a copy in this version's format.

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

sources/qetdiagrameditor.cpp

@@ -1969,6 +1969,16 @@ bool QETDiagramEditor::openAndAddProject(
 	addProject(project);
 	DialogWaiting::dropInstance();
 
+	if (interactive && project->isFromLaterVersion()) {
+		QET::QetMessageBox::warning(
+			this,
+			tr("Opening the file read-only", "message box title"),
+			tr("This project was saved by a later version of QElectroTech. "
+			   "It is opened read-only, so that saving it cannot lose what "
+			   "this version does not know. Use Save As to keep a copy "
+			   "in this version's format."));
+	}
+
 		//Report font descriptions which could not be read as-is (written by
 		//an incompatible Qt version or corrupted), so the user learns about
 		//it from somewhere else than the console. See issue #553.