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.

android/src/main/kotlin/io/swipelab/ux/GalleryPlugin.kt

@@ -6,15 +6,19 @@ import android.content.ContentUris
 import android.content.Context
 import android.content.Intent
 import android.content.pm.PackageManager
+import android.database.ContentObserver
 import android.database.Cursor
 import android.net.Uri
 import android.os.Build
+import android.os.Handler
+import android.os.Looper
 import android.provider.MediaStore
 import android.provider.Settings
 import android.util.Size
 import androidx.core.content.ContextCompat
 import io.flutter.embedding.engine.plugins.FlutterPlugin
 import io.flutter.embedding.engine.plugins.activity.ActivityPluginBinding
+import io.flutter.plugin.common.EventChannel
 import io.flutter.plugin.common.MethodCall
 import io.flutter.plugin.common.MethodChannel
 import io.flutter.plugin.common.PluginRegistry
@@ -28,8 +32,9 @@ import java.io.FileOutputStream
 /// resolution into the app cache so `dart:io` can read what the
 /// system holds behind a `content://` URI.
 class GalleryPlugin : NativePlugin, MethodChannel.MethodCallHandler,
-    PluginRegistry.RequestPermissionsResultListener {
+    PluginRegistry.RequestPermissionsResultListener, EventChannel.StreamHandler {
     private var methodChannel: MethodChannel? = null
+    private var changesChannel: EventChannel? = null
     private var context: Context? = null
     private var activity: Activity? = null
     private var activityBinding: ActivityPluginBinding? = null
@@ -39,19 +44,64 @@ class GalleryPlugin : NativePlugin, MethodChannel.MethodCallHandler,
     /// is rejected.
     private var pendingPermissionResult: MethodChannel.Result? = null
 
+    /// Dart subscriber on `ux/gallery/changes` — fed by [mediaObserver]
+    /// when `MediaStore` reports an insert/update/delete so picker UIs
+    /// can reload reactively (parity with iOS's `photoLibraryDidChange`).
+    private var libraryEventSink: EventChannel.EventSink? = null
+    private val mainHandler = Handler(Looper.getMainLooper())
+    private val mediaObserver = object : ContentObserver(mainHandler) {
+        override fun onChange(selfChange: Boolean, uri: Uri?) {
+            libraryEventSink?.success(null)
+        }
+    }
+    private var mediaObserverRegistered = false
+
     override fun onAttachedToEngine(binding: FlutterPlugin.FlutterPluginBinding) {
         context = binding.applicationContext
         methodChannel = MethodChannel(binding.binaryMessenger, "ux/gallery").also {
             it.setMethodCallHandler(this)
         }
+        changesChannel = EventChannel(
+            binding.binaryMessenger, "ux/gallery/changes",
+        ).also { it.setStreamHandler(this) }
     }
 
     override fun onDetachedFromEngine(binding: FlutterPlugin.FlutterPluginBinding) {
         methodChannel?.setMethodCallHandler(null)
         methodChannel = null
+        changesChannel?.setStreamHandler(null)
+        changesChannel = null
+        unregisterMediaObserver()
         context = null
     }
 
+    override fun onListen(arguments: Any?, events: EventChannel.EventSink?) {
+        libraryEventSink = events
+        registerMediaObserver()
+    }
+
+    override fun onCancel(arguments: Any?) {
+        libraryEventSink = null
+        unregisterMediaObserver()
+    }
+
+    private fun registerMediaObserver() {
+        if (mediaObserverRegistered) return
+        val ctx = context ?: return
+        ctx.contentResolver.registerContentObserver(
+            MediaStore.Files.getContentUri(MediaStore.VOLUME_EXTERNAL),
+            /* notifyForDescendants */ true,
+            mediaObserver,
+        )
+        mediaObserverRegistered = true
+    }
+
+    private fun unregisterMediaObserver() {
+        if (!mediaObserverRegistered) return
+        context?.contentResolver?.unregisterContentObserver(mediaObserver)
+        mediaObserverRegistered = false
+    }
+
     override fun onAttachedToActivity(binding: ActivityPluginBinding) {
         activity = binding.activity
         activityBinding = binding