Reflect the video below the hinge in the tabletop layout

The iPhone Duo tabletop layout mirrors every presented frame into the
lower half, blurred and fading out, like a screen standing on a glossy
table. The render view blits a quarter-resolution copy of the display
framebuffer into a ring of IOSurface-backed buffers (no Y flip, so the
surface is already mirrored) and hands it to a CALayer hosted by
MPVReflectionView; the simulator's software renderer feeds the same
layer a flipped CGImage. Off during PiP, for audio-only streams, and
via the new Playback → Foldable → "Reflection Below Video" setting.

Yattee/Views/Player/MPVReflectionView.swift

@@ -0,0 +1,79 @@
+//
+//  MPVReflectionView.swift
+//  Yattee
+//
+//  Hosts the backend's mirrored frame layer below the video in the iPhone
+//  Duo tabletop layout, so the lower half shows a dim "reflection" of the
+//  upper half like a screen standing on a glossy table.
+//
+
+import SwiftUI
+
+#if os(iOS)
+import UIKit
+
+/// Shows the mirrored video frames the MPV render view captures while the
+/// view is on screen. Sized to the fitted video; place it directly below the
+/// hinge line and fade it with `tabletopReflectionOverlay`.
+struct MPVReflectionView: UIViewRepresentable {
+    let backend: MPVBackend
+
+    func makeUIView(context: Context) -> ReflectionHostView {
+        let view = ReflectionHostView()
+        view.backend = backend
+        view.attach(backend.reflectionLayer)
+        backend.setReflectionEnabled(true)
+        return view
+    }
+
+    func updateUIView(_ uiView: ReflectionHostView, context: Context) {
+        // The backend's player view can be recreated; re-attach the current layer.
+        uiView.backend = backend
+        uiView.attach(backend.reflectionLayer)
+        backend.setReflectionEnabled(true)
+    }
+
+    static func dismantleUIView(_ uiView: ReflectionHostView, coordinator: ()) {
+        uiView.backend?.setReflectionEnabled(false)
+        uiView.attach(nil)
+    }
+
+    final class ReflectionHostView: UIView {
+        private weak var hostedLayer: CALayer?
+        weak var backend: MPVBackend?
+
+        override init(frame: CGRect) {
+            super.init(frame: frame)
+            backgroundColor = .clear
+            isUserInteractionEnabled = false
+        }
+
+        required init?(coder: NSCoder) {
+            super.init(coder: coder)
+        }
+
+        func attach(_ newLayer: CALayer?) {
+            guard newLayer !== hostedLayer else { return }
+            hostedLayer?.removeFromSuperlayer()
+            hostedLayer = newLayer
+            if let newLayer {
+                newLayer.frame = bounds
+                layer.addSublayer(newLayer)
+            }
+        }
+
+        override func layoutSubviews() {
+            super.layoutSubviews()
+            CATransaction.begin()
+            CATransaction.setDisableActions(true)
+            hostedLayer?.frame = bounds
+            CATransaction.commit()
+        }
+
+        deinit {
+            hostedLayer?.removeFromSuperlayer()
+        }
+    }
+}
+
+#endif