gallery: library-change Stream + native observer parity

Add UxGallery.libraryChanges — a Stream<void> that emits whenever the
underlying photo library reports a change, so picker UIs can drop
their cached asset lists and reload reactively.

- iOS: GalleryPlugin conforms to PHPhotoLibraryChangeObserver +
  FlutterStreamHandler; on photoLibraryDidChange the fetchCache is
  cleared and a void event is pushed over ux/gallery/changes. The
  observer is registered lazily on the first granted/limited
  authorization so plugin init doesn't trigger iOS's permission
  evaluation at app launch.
- macOS: near-verbatim port of the iOS shape (same Photos.framework,
  same fetchCache staleness, same fix).
- Android: registers a MediaStore ContentObserver on
  Files.getContentUri(VOLUME_EXTERNAL) with notifyForDescendants =
  true; observer lifecycle tracks the Dart subscription (registered
  on onListen, unregistered on onCancel / plugin detach). No native
  cache to invalidate today, but the pipeline is wired for the
  upcoming READ_MEDIA_VISUAL_USER_SELECTED limited-access work.
- iOS presentLimitedLibraryPicker switched to the iOS 15+ completion-
  handler variant so the Dart await resolves on dismissal, not on
  presentation. The actual reload is now driven by the change
  observer (which fires after iOS commits the new subset),
  side-stepping the completion-vs-commit race that produced an
  off-by-one in the picker on consecutive MANAGE taps.
- FakeUxGalleryBackend exposes emitLibraryChange() so tests can
  drive the reactive-reload wiring without going through the real
  method channel.

macos/Classes/GalleryPlugin.swift

@@ -5,9 +5,14 @@ import Photos
 /// macOS counterpart of the iOS gallery bridge — same `Photos.framework`
 /// data layer, with `NSImage` swapped in for `UIImage` and the
 /// limited-library picker dropped (macOS has no equivalent).
-public class GalleryPlugin: NSObject, NativePlugin {
+public class GalleryPlugin: NSObject, NativePlugin, PHPhotoLibraryChangeObserver, FlutterStreamHandler {
     private let imageManager = PHCachingImageManager()
     private var fetchCache: [String: PHFetchResult<PHAsset>] = [:]
+    private var libraryObserverRegistered = false
+
+    /// Sink for the `ux/gallery/changes` event channel — emits void on
+    /// every `photoLibraryDidChange` so picker UIs can reload reactively.
+    private var libraryEventSink: FlutterEventSink?
 
     public func register(with registrar: FlutterPluginRegistrar) {
         let channel = FlutterMethodChannel(
@@ -17,15 +22,68 @@ public class GalleryPlugin: NSObject, NativePlugin {
         channel.setMethodCallHandler { [weak self] call, result in
             self?.handle(call: call, result: result)
         }
+        let events = FlutterEventChannel(
+            name: "ux/gallery/changes",
+            binaryMessenger: registrar.messenger,
+        )
+        events.setStreamHandler(self)
+    }
+
+    public func onListen(
+        withArguments arguments: Any?,
+        eventSink events: @escaping FlutterEventSink,
+    ) -> FlutterError? {
+        libraryEventSink = events
+        return nil
+    }
+
+    public func onCancel(withArguments arguments: Any?) -> FlutterError? {
+        libraryEventSink = nil
+        return nil
+    }
+
+    /// Photos delivers change events only after the user grants access;
+    /// defer registration until then so plugin init doesn't poke the
+    /// permission machinery at app launch.
+    private func ensureLibraryObserver() {
+        guard !libraryObserverRegistered else { return }
+        PHPhotoLibrary.shared().register(self)
+        libraryObserverRegistered = true
+    }
+
+    deinit {
+        if libraryObserverRegistered {
+            PHPhotoLibrary.shared().unregisterChangeObserver(self)
+        }
+    }
+
+    public func photoLibraryDidChange(_ changeInstance: PHChange) {
+        DispatchQueue.main.async { [weak self] in
+            self?.fetchCache.removeAll()
+            self?.libraryEventSink?(nil)
+        }
+    }
+
+    private static func isAuthorizedForLibrary(_ status: PHAuthorizationStatus) -> Bool {
+        if status == .authorized { return true }
+        if #available(macOS 11.0, *), status == .limited { return true }
+        return false
     }
 
     private func handle(call: FlutterMethodCall, result: @escaping FlutterResult) {
         switch call.method {
         case "permission":
-            result(Self.permissionString(Self.currentAuthorization()))
+            let status = Self.currentAuthorization()
+            if Self.isAuthorizedForLibrary(status) {
+                ensureLibraryObserver()
+            }
+            result(Self.permissionString(status))
         case "requestPermission":
-            Self.requestAuthorization { status in
+            Self.requestAuthorization { [weak self] status in
                 DispatchQueue.main.async {
+                    if Self.isAuthorizedForLibrary(status) {
+                        self?.ensureLibraryObserver()
+                    }
                     result(Self.permissionString(status))
                 }
             }