docs(gtk4): add porting TODOs

docs/gtk4_porting_todos.md

@@ -0,0 +1,38 @@
+# GTK4 Porting TODOs
+
+This checklist maps remaining work to GTK4 migration guide changes and to
+specific files in this repo.
+
+## Window lifecycle/state (GdkToplevel)
+- Replace GTK3 `window-state-event`/`GdkWindowState` with GTK4 `GdkToplevelState`
+  updates in `engine/src/flutter/shell/platform/linux/fl_window_state_monitor.cc`.
+- Ensure lifecycle mapping:
+  - visible + focused => `AppLifecycleState.resumed`
+  - visible + not focused => `AppLifecycleState.inactive`
+  - not visible/minimized => `AppLifecycleState.hidden`
+
+## Window configure/state notifications (GTK4 signals)
+- Replace GTK3 `configure-event` and `window-state-event` in
+  `engine/src/flutter/shell/platform/linux/fl_window_monitor.cc` with GTK4
+  equivalents (toplevel state or surface size/notify signals).
+
+## Accessibility (ATK removal)
+- Replace ATK-based implementation with GTK4 `GtkAccessible` API or
+  compatible bridge:
+  - `engine/src/flutter/shell/platform/linux/fl_view.cc`
+  - `engine/src/flutter/shell/platform/linux/fl_accessibility_handler.cc`
+  - `engine/src/flutter/shell/platform/linux/fl_view_accessible.*` and
+    related ATK types
+
+## GTK4 test coverage
+- GTK3-specific tests are disabled under `use_gtk4`. Create GTK4 tests/mocks:
+  - `engine/src/flutter/shell/platform/linux/fl_window_state_monitor_test.cc`
+  - `engine/src/flutter/shell/platform/linux/testing/mock_gtk.*`
+
+## Runner templates (already updated)
+- GTK4 runner templates now use GTK4 APIs and pkg-config:
+  - `packages/flutter_tools/templates/app/linux-gtk4.tmpl/*`
+
+## Plugin templates (GTK4)
+- Plugin templates use GTK4 but may still need review for deprecated APIs:
+  - `packages/flutter_tools/templates/plugin/linux-gtk4.tmpl/*`