Keep wires attached when a project's copy of a symbol is replaced

A wire whose terminals have uuids is saved against them, and on load it
is reattached to a terminal with that uuid or dropped, with only a qDebug
line. Re-importing a changed symbol and choosing "replace" swapped the
project's definition for one whose terminal uuids differ; the placed
symbols kept the old ones until the project was reopened, so every wire
on them was lost at the next open, silently, and gone for good at the
next save.

- XmlElementCollection::copyElement(), where an embedded definition is
  overwritten, carries each old terminal uuid onto the new terminal at
  the same place and orientation (TerminalUuids::keep()). Terminals that
  moved, and new ones, keep their own.
- Diagram::fromXml() records wires it could not reattach, logs them, and
  the editor lists them in one warning after opening a project.

Measured on 2612_ats_singlephase.qet with the stored splice's terminal
uuids made to differ from the collection's: replace, save, reopen loads
34 of 131 wires on master, 131 with this change (GUI, both arms).
tst_terminaluuids covers keep() and runs the real loader.

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

sources/qetdiagrameditor.cpp

@@ -1672,6 +1672,34 @@ bool QETDiagramEditor::openAndAddProject(
 		);
 	}
 
+		//Report wires left out of the load because a terminal they join
+		//was not found: they would otherwise vanish on the next save
+		//without the user ever being told.
+	QStringList lost_wires;
+	for (Diagram *diagram : project->diagrams()) {
+		for (const QString &wire : diagram->wiresNotReconnected()) {
+			lost_wires << tr("Folio %1 : %2").arg(diagram->folioIndex() + 1).arg(wire);
+		}
+	}
+	if (interactive && !lost_wires.isEmpty())
+	{
+		QMessageBox box(QMessageBox::Warning,
+						tr("Conducteurs non chargés", "message box title"),
+						tr("%n conducteur(s) n'ont pas pu être reliés à leurs"
+						   " bornes et n'ont pas été chargés. La définition de"
+						   " l'élément dans le projet a probablement été remplacée"
+						   " par une autre dont les bornes diffèrent.\n\n"
+						   "Si vous enregistrez le projet, ces conducteurs"
+						   " disparaîtront du fichier. Fermez-le sans enregistrer pour"
+						   " conserver le fichier tel quel.",
+						   "message box content",
+						   lost_wires.size()),
+						QMessageBox::Ok,
+						this);
+		box.setDetailedText(lost_wires.join(QLatin1Char('\n')));
+		box.exec();
+	}
+
 	BackupDialog backup_dialog(this);
 	if (backup_dialog.exec() == QDialog::Accepted)
 	{