Add regression test for elements-panel project-open auto-select crash

- tst_elementspanelprojectopen drives ElementsPanel::projectWasOpened() --
  the slot the application connects to a project being opened -- on a fresh
  panel whose first reload() has not run, reproducing the detached-item
  ordering deterministically offscreen.
- On master without the guard the test fails: the auto-select leaves
  currentItem() pointing at an item whose treeWidget() is not the panel (and
  on some builds crashes in QTreeModel). With the guard it passes: no item is
  selected while the project is detached.
- Registered via qet_add_app_test(), which links the application code and
  runs a real QETApp offscreen.

Generated-by: Claude Code
Signed-off-by: hairykiwi <174199+hairykiwi@users.noreply.github.com>

tests/qttest/tst_elementspanelprojectopen.cpp

@@ -0,0 +1,44 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+#include "qetapptest.h"
+
+#include "elementspanel.h"
+#include "projectview.h"
+#include "qetproject.h"
+
+// projectWasOpened() adds the project to the elements panel. Before the panel's
+// first reload() the project's tree item is left detached (treeWidget() ==
+// nullptr), and selecting its first child crashed in QTreeModel
+// (pre-existing since a82f6de23).
+class tst_elementspanelprojectopen : public QObject
+{
+	Q_OBJECT
+
+	private slots:
+		void addProjectBeforeReloadDoesNotCrash()
+		{
+				// A real one-folio project from the running app; diagrams()
+				// builds the lazy folio, so the project item will have a child.
+			QETDiagramEditor *editor = QetAppTest::editor();
+			QVERIFY(editor);
+			QVERIFY(editor->newProject());
+			ProjectView *pv = editor->currentProjectView();
+			QVERIFY(pv);
+			QETProject *project = pv->project();
+			QVERIFY(project);
+			QVERIFY(!project->diagrams().isEmpty());
+
+				// Fresh panel: reload() has not run, so the project item is inserted detached.
+			ElementsPanel panel;
+
+				// Without the guard this selects a detached item; with it, nothing is selected.
+			panel.projectWasOpened(project);
+
+			QTreeWidgetItem *current = panel.currentItem();
+			QVERIFY2(current == nullptr || current->treeWidget() == &panel,
+				"the panel selected a detached item it does not own");
+		}
+};
+
+QET_APP_TEST_MAIN(tst_elementspanelprojectopen)
+
+#include "tst_elementspanelprojectopen.moc"