Route app shutdown staging through app core

tests/app_core/app_shutdown_tests.cpp

@@ -0,0 +1,29 @@
+#include "app_core/app_shutdown.h"
+#include "test_harness.h"
+
+namespace {
+
+void shutdown_plan_preserves_legacy_cleanup_sequence(pp::tests::Harness& harness)
+{
+    const auto plan = pp::app::plan_app_shutdown();
+
+    PP_EXPECT(harness, plan.save_ui_state);
+    PP_EXPECT(harness, plan.terminate_stroke_preview_renderer);
+    PP_EXPECT(harness, plan.stop_recording);
+    PP_EXPECT(harness, plan.invalidate_textures);
+    PP_EXPECT(harness, plan.invalidate_shaders);
+    PP_EXPECT(harness, plan.unload_layouts);
+    PP_EXPECT(harness, plan.destroy_ui_render_target);
+    PP_EXPECT(harness, plan.destroy_face_plane);
+    PP_EXPECT(harness, plan.release_panel_nodes);
+    PP_EXPECT(harness, plan.clear_quick_mode_state);
+}
+
+} // namespace
+
+int main()
+{
+    pp::tests::Harness harness;
+    harness.run("shutdown plan preserves legacy cleanup sequence", shutdown_plan_preserves_legacy_cleanup_sequence);
+    return harness.finish();
+}