video_player + insets: native playback backend + animated viewPadding

- video_player: ExoPlayer (Android) / AVPlayer (iOS/macOS) backend with
  PixelBufferSink, method-channel adapter, Dart-side XVideoPlayer +
  testing fake.
- insets: XInsets singleton + XAnimatedInsets widget lerp the system
  viewPadding over 220ms so OS bar visibility toggles
  (immersiveSticky <-> edgeToEdge) slide bottom-/top-anchored UI into
  place instead of snapping by the nav-bar / status-bar height.

ios/Classes/XPlugin.swift

@@ -14,6 +14,7 @@ public class XPlugin: NSObject, FlutterPlugin {
             GalleryPlugin(),
             CrashPlugin(),
             CameraPlugin(),
+            UxVideoPlayerPlugin(),
             UrlPlugin(),
         ]
         for plugin in plugins {