notifications + window: add Android native plugins

`ux/notifications/events` and `ux/window/events` only had macOS stream
handlers, so on Android/iOS the unconditional Dart subscription threw
MissingPluginException at startup (EventChannel reports activation
failures straight to FlutterError.onError, bypassing the `onError:`
callback).

- Gate each Dart event-channel subscription to platforms that register a
  native handler (`defaultTargetPlatform`), silencing iOS.
- `WindowPlugin`: report app foreground/background as host focus via
  `ProcessLifecycleOwner` ON_START/ON_STOP, so a backgrounded-but-alive
  process reports `focused = false`.
- `NotificationsPlugin`: local notifications (show/cancel by thread/all),
  POST_NOTIFICATIONS request on 13+, and tap routing back over the event
  channel — a tap that cold-starts the process is buffered until Dart
  subscribes.
- Regression tests for the subscription gate plus contract tests for the
  method/event payloads.

test/window_test.dart

@@ -0,0 +1,63 @@
+import 'package:flutter/foundation.dart';
+import 'package:flutter/services.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:ux/ux.dart';
+
+void main() {
+  final messenger =
+      TestWidgetsFlutterBinding.ensureInitialized().defaultBinaryMessenger;
+  const channel = MethodChannel('ux/window/events');
+
+  final listenCalls = <String>[];
+
+  setUp(() {
+    listenCalls.clear();
+    messenger.setMockMethodCallHandler(channel, (call) async {
+      listenCalls.add(call.method);
+      return null;
+    });
+  });
+
+  tearDown(() {
+    messenger.setMockMethodCallHandler(channel, null);
+    debugDefaultTargetPlatformOverride = null;
+  });
+
+  Future<void> sendEvent(Object? event) => messenger.handlePlatformMessage(
+        'ux/window/events',
+        const StandardMethodCodec().encodeSuccessEnvelope(event),
+        (_) {},
+      );
+
+  // Regression: iOS has no native handler, so activating the broadcast
+  // stream throws MissingPluginException straight to FlutterError.onError.
+  test('iOS does not activate the events channel', () async {
+    debugDefaultTargetPlatformOverride = TargetPlatform.iOS;
+    final w = MethodChannelXWindow();
+    await pumpEventQueue();
+    expect(listenCalls, isEmpty);
+    expect(w.focused.value, isTrue);
+  });
+
+  test('Android activates the events channel', () async {
+    debugDefaultTargetPlatformOverride = TargetPlatform.android;
+    MethodChannelXWindow();
+    await pumpEventQueue();
+    expect(listenCalls, contains('listen'));
+  });
+
+  test('a focus event flips focused', () async {
+    debugDefaultTargetPlatformOverride = TargetPlatform.android;
+    final w = MethodChannelXWindow();
+    await pumpEventQueue();
+    expect(w.focused.value, isTrue);
+
+    await sendEvent({'type': 'focus', 'focused': false});
+    await pumpEventQueue();
+    expect(w.focused.value, isFalse);
+
+    await sendEvent({'type': 'focus', 'focused': true});
+    await pumpEventQueue();
+    expect(w.focused.value, isTrue);
+  });
+}