Route app shutdown staging through app core

src/app_core/app_shutdown.h

@@ -0,0 +1,23 @@
+#pragma once
+
+namespace pp::app {
+
+struct AppShutdownPlan {
+    bool save_ui_state = true;
+    bool terminate_stroke_preview_renderer = true;
+    bool stop_recording = true;
+    bool invalidate_textures = true;
+    bool invalidate_shaders = true;
+    bool unload_layouts = true;
+    bool destroy_ui_render_target = true;
+    bool destroy_face_plane = true;
+    bool release_panel_nodes = true;
+    bool clear_quick_mode_state = true;
+};
+
+[[nodiscard]] constexpr AppShutdownPlan plan_app_shutdown() noexcept
+{
+    return AppShutdownPlan {};
+}
+
+} // namespace pp::app