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.

lib/src/notifications/x_notifications.dart

@@ -1,6 +1,7 @@
 import 'dart:async';
 
-import 'package:flutter/foundation.dart' show ValueListenable;
+import 'package:flutter/foundation.dart'
+    show ValueListenable, defaultTargetPlatform, TargetPlatform;
 import 'package:flutter/services.dart';
 
 import '../core/emitter.dart';
@@ -9,9 +10,10 @@ import '../log.dart' show Log;
 final _log = Log.tag('notifications');
 
 /// OS-notification primitive. Domain-agnostic — the caller decides when
-/// to emit and how to format the payload. Implemented on macOS via
-/// `UNUserNotificationCenter` (see `macos/Classes/NotificationsPlugin.swift`).
-/// Window-focus state lives in [XWindow], not here.
+/// to emit and how to format the payload. Backed natively on macOS
+/// (`UNUserNotificationCenter`) and Android (`NotificationManagerCompat`);
+/// see the platform `NotificationsPlugin`. No handler on iOS, where calls
+/// no-op. Window-focus state lives in [XWindow], not here.
 abstract interface class XNotifications {
   /// Production singleton — talks to the native plugin via MethodChannel
   /// + EventChannel. Tests inject a fake `XNotifications` directly into
@@ -38,11 +40,18 @@ abstract interface class XNotifications {
 
 class MethodChannelXNotifications implements XNotifications {
   MethodChannelXNotifications() {
+    if (!_hasNativeHandler) return;
     _eventsChannel.receiveBroadcastStream().listen(_onEvent, onError: (e, st) {
       _log.w('event channel error', error: e, stackTrace: st);
     });
   }
 
+  /// Only macOS and Android register a native handler; activating the events
+  /// channel elsewhere throws `MissingPluginException`.
+  static bool get _hasNativeHandler =>
+      defaultTargetPlatform == TargetPlatform.macOS ||
+      defaultTargetPlatform == TargetPlatform.android;
+
   static const _channel = MethodChannel('ux/notifications');
   static const _eventsChannel = EventChannel('ux/notifications/events');
 
@@ -90,8 +99,7 @@ class MethodChannelXNotifications implements XNotifications {
   @override
   Future<void> cancelByThread(String threadId) async {
     try {
-      await _channel
-          .invokeMethod('cancelByThread', {'threadId': threadId});
+      await _channel.invokeMethod('cancelByThread', {'threadId': threadId});
     } on PlatformException catch (e) {
       _log.w('cancelByThread failed: ${e.message}');
     } on MissingPluginException {

lib/src/window/x_window.dart

@@ -1,6 +1,5 @@
-import 'dart:async';
-
-import 'package:flutter/foundation.dart' show ValueListenable;
+import 'package:flutter/foundation.dart'
+    show ValueListenable, defaultTargetPlatform, TargetPlatform;
 import 'package:flutter/services.dart';
 
 import '../core/emitter.dart';
@@ -17,22 +16,28 @@ abstract interface class XWindow {
 
   /// `true` when this app currently has user focus.
   ///
-  /// - macOS: tracks `NSApplication.didBecomeActiveNotification` /
-  ///   `didResignActiveNotification` via the native plugin.
-  /// - iOS / Android: no native plugin registers, so the emitter stays
-  ///   at its `true` default. That's correct because socket frames only
-  ///   reach the Dart isolate while the process is alive — by the time
-  ///   the OS suspends the app, deliveries have stopped.
+  /// - macOS: tracks `NSApplication` active / resign via the native plugin.
+  /// - Android: tracks app foreground / background via `ProcessLifecycleOwner`,
+  ///   so a backgrounded-but-alive process reports `false`.
+  /// - iOS: no native plugin registers, so the emitter stays at its `true`
+  ///   default.
   ValueListenable<bool> get focused;
 }
 
 class MethodChannelXWindow implements XWindow {
   MethodChannelXWindow() {
+    if (!_hasNativeHandler) return;
     _events.receiveBroadcastStream().listen(_onEvent, onError: (e, st) {
       _log.w('event channel error', error: e, stackTrace: st);
     });
   }
 
+  /// Only macOS and Android register a native stream handler for the events
+  /// channel; activating it elsewhere throws `MissingPluginException`.
+  static bool get _hasNativeHandler =>
+      defaultTargetPlatform == TargetPlatform.macOS ||
+      defaultTargetPlatform == TargetPlatform.android;
+
   static const _events = EventChannel('ux/window/events');
 
   @override