window: extract XWindow primitive; XNotifications stops carrying focus

The window-focus signal had no business living on the notifications
primitive — it was there because the same NotificationsPlugin happened
to observe NSApplication active/resign for its own reasons. Splitting
it into a sibling XWindow primitive (with its own WindowPlugin on
macOS, ux/window/events) lets future consumers — paused video,
deferred-work scheduling, dock badge counts — read focus state without
pulling in UNUserNotificationCenter.

XNotifications now only exposes notification I/O (show/cancel + tap +
authorization). The 'type:focus' event-channel branch is gone.

macos/Classes/WindowPlugin.swift

@@ -0,0 +1,68 @@
+import AppKit
+import FlutterMacOS
+
+/// `ux/window/events`. Reports host-application focus state — emits
+/// `{type: "focus", focused: Bool}` whenever the active NSApplication
+/// flips. Seeded from `NSApp.isActive` when Dart subscribes so the
+/// emitter starts in a deterministic value.
+public class WindowPlugin: NSObject, NativePlugin, FlutterStreamHandler {
+    private var eventChannel: FlutterEventChannel?
+    private var eventSink: FlutterEventSink?
+
+    public func register(with registrar: FlutterPluginRegistrar) {
+        let events = FlutterEventChannel(
+            name: "ux/window/events",
+            binaryMessenger: registrar.messenger,
+        )
+        events.setStreamHandler(self)
+        eventChannel = events
+
+        observeAppActivation()
+    }
+
+    // MARK: - FlutterStreamHandler
+
+    public func onListen(
+        withArguments arguments: Any?,
+        eventSink events: @escaping FlutterEventSink
+    ) -> FlutterError? {
+        eventSink = events
+        emit(["type": "focus", "focused": NSApp.isActive])
+        return nil
+    }
+
+    public func onCancel(withArguments arguments: Any?) -> FlutterError? {
+        eventSink = nil
+        return nil
+    }
+
+    // MARK: - Observers
+
+    private func observeAppActivation() {
+        let nc = NotificationCenter.default
+        nc.addObserver(
+            self,
+            selector: #selector(onDidBecomeActive),
+            name: NSApplication.didBecomeActiveNotification,
+            object: nil,
+        )
+        nc.addObserver(
+            self,
+            selector: #selector(onDidResignActive),
+            name: NSApplication.didResignActiveNotification,
+            object: nil,
+        )
+    }
+
+    @objc private func onDidBecomeActive() {
+        emit(["type": "focus", "focused": true])
+    }
+
+    @objc private func onDidResignActive() {
+        emit(["type": "focus", "focused": false])
+    }
+
+    private func emit(_ event: [String: Any]) {
+        eventSink?(event)
+    }
+}