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/Video-shared/VideoPixelBufferSink.swift

@@ -0,0 +1,55 @@
+import CoreVideo
+import Foundation
+#if canImport(UIKit)
+import Flutter
+#else
+import FlutterMacOS
+#endif
+
+/// Single-slot latest-pixel-buffer sink that feeds a `FlutterTexture`.
+/// Mirrors `PreviewSink` in `darwin/Camera/` — receiver writes, the
+/// engine reads via `copyPixelBuffer()`. We retain only the most
+/// recent buffer; older ones get released the moment a new frame
+/// arrives, bounding memory at one frame.
+final class UxVideoPixelBufferSink: NSObject, FlutterTexture {
+    private weak var registry: FlutterTextureRegistry?
+    private var textureId: Int64 = -1
+
+    private let bufferQueue = DispatchQueue(
+        label: "ux.video.buffer",
+        qos: .userInitiated
+    )
+    private var latestPixelBuffer: CVPixelBuffer?
+
+    func register(with registry: FlutterTextureRegistry) -> Int64 {
+        self.registry = registry
+        textureId = registry.register(self)
+        return textureId
+    }
+
+    func unregister() {
+        registry?.unregisterTexture(textureId)
+        bufferQueue.sync { latestPixelBuffer = nil }
+    }
+
+    // MARK: - FlutterTexture
+
+    func copyPixelBuffer() -> Unmanaged<CVPixelBuffer>? {
+        var pb: CVPixelBuffer?
+        bufferQueue.sync {
+            pb = latestPixelBuffer
+            latestPixelBuffer = nil
+        }
+        if let pb = pb { return Unmanaged.passRetained(pb) }
+        return nil
+    }
+
+    /// Receives a new frame from the AVPlayerItemVideoOutput pump.
+    /// Cheap — just swaps the pointer + notifies the registry.
+    func deliver(pixelBuffer: CVPixelBuffer) {
+        bufferQueue.sync { latestPixelBuffer = pixelBuffer }
+        if let registry = registry {
+            registry.textureFrameAvailable(textureId)
+        }
+    }
+}