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.

android/build.gradle

@@ -70,4 +70,10 @@ dependencies {
     implementation 'androidx.lifecycle:lifecycle-process:2.7.0'
     // Pure-Kotlin/Java QR decoder. ~470 KB jar, no Play Services dep.
     implementation 'com.google.zxing:core:3.5.3'
+    // Media3 for ux.video_player. Same version line video_player_android
+    // 2.9.5 pulls in (1.9.x) so the spike fork can coexist during the
+    // Phase 2/3 migration without dragging in two ExoPlayer copies.
+    def media3Version = '1.9.2'
+    implementation "androidx.media3:media3-exoplayer:$media3Version"
+    implementation "androidx.media3:media3-common:$media3Version"
 }