fix(player): render HDR letterbox bars off the SDR window plane

HDR/Dolby Vision playback on Fire TV (and some Sony/Philips models)
shows gray letterbox bars instead of black on OLED panels: the bars
were the player container's window-plane background, and those device
compositors raise SDR graphics-plane black while the display is in
HDR/DV output mode. Black inside the video plane is unaffected (same
mechanism as ExoPlayer #8803 and Kodi #25300).

Add a fullscreen, buffer-less SurfaceView beneath the video surface in
the shared player container. Like any below-window SurfaceView it
registers its rect as a window transparent region, so the letterbox
area scans out as the SurfaceFlinger backdrop instead of window-plane
pixels. No buffer is ever posted: SurfaceFlinger skips buffer-less
layers, and drawing into one would put the bars back on an SDR layer.
Covers both the ExoPlayer and mpv backends.

close #2163

android/app/src/main/kotlin/com/edde746/plezy/shared/PlayerSurfaceHost.kt

@@ -15,8 +15,37 @@ internal object PlayerSurfaceHost {
       ViewGroup.LayoutParams.MATCH_PARENT,
       ViewGroup.LayoutParams.MATCH_PARENT
     )
+    // Fallback fill for the frames before the punch surface below is placed.
     setBackgroundColor(Color.BLACK)
     this.clipChildren = clipChildren
+    addView(createLetterboxPunchSurface(activity))
+  }
+
+  /**
+   * Fullscreen, buffer-less SurfaceView kept beneath the video surface.
+   *
+   * Its only job is taking the letterbox area off the app-window (graphics)
+   * plane: like any below-window SurfaceView it punches the parent canvas and
+   * registers its rect as a window transparent region, so the pixels around
+   * the video rect scan out as the SurfaceFlinger backdrop instead of
+   * window-plane black. Several TV compositors (Fire TV Stick 4K Max, some
+   * Sony/Philips models) raise SDR graphics-plane black while the display is
+   * in HDR/Dolby Vision mode, which shows as gray letterbox bars on OLED
+   * panels (issue #2163; same mechanism as ExoPlayer #8803 and Kodi #25300).
+   *
+   * No buffer is ever posted to it: SurfaceFlinger skips buffer-less layers,
+   * and drawing black into it would put the bars back onto an SDR layer —
+   * exactly the plane being avoided. Views drawn on the parent canvas after
+   * the punch (Media3 subtitle cues) still re-claim their own bounds.
+   */
+  private fun createLetterboxPunchSurface(activity: Activity): SurfaceView = SurfaceView(activity).apply {
+    layoutParams = FrameLayout.LayoutParams(
+      FrameLayout.LayoutParams.MATCH_PARENT,
+      FrameLayout.LayoutParams.MATCH_PARENT
+    )
+    setZOrderOnTop(false)
+    setZOrderMediaOverlay(false)
+    FlutterOverlayHelper.applyCompositionOrder(this, -2)
   }
 
   fun createVideoSurface(activity: Activity, callback: SurfaceHolder.Callback): SurfaceView = SurfaceView(activity).apply {

android/app/src/test/kotlin/com/edde746/plezy/shared/PlayerSurfaceHostTest.kt

@@ -4,6 +4,7 @@ import android.app.Activity
 import android.graphics.Color
 import android.graphics.drawable.ColorDrawable
 import android.view.SurfaceHolder
+import android.view.SurfaceView
 import android.view.ViewGroup
 import android.widget.FrameLayout
 import org.junit.Assert.assertEquals
@@ -41,4 +42,27 @@ class PlayerSurfaceHostTest {
     assertEquals(FrameLayout.LayoutParams.MATCH_PARENT, surface.layoutParams.width)
     assertEquals(FrameLayout.LayoutParams.MATCH_PARENT, surface.layoutParams.height)
   }
+
+  @Test
+  fun letterboxAreaIsCoveredByABufferlessPunchSurfaceBeneathTheVideo() {
+    val activity = Robolectric.buildActivity(Activity::class.java).setup().get()
+    activity.setContentView(FrameLayout(activity))
+    val callback = object : SurfaceHolder.Callback {
+      override fun surfaceCreated(holder: SurfaceHolder) = Unit
+      override fun surfaceChanged(holder: SurfaceHolder, format: Int, width: Int, height: Int) = Unit
+      override fun surfaceDestroyed(holder: SurfaceHolder) = Unit
+    }
+
+    val container = PlayerSurfaceHost.createContainer(activity)
+    val punch = container.getChildAt(0)
+    assertTrue(punch is SurfaceView)
+    assertEquals(FrameLayout.LayoutParams.MATCH_PARENT, punch.layoutParams.width)
+    assertEquals(FrameLayout.LayoutParams.MATCH_PARENT, punch.layoutParams.height)
+
+    // Cores append their surfaces after createContainer, so video (and the mpv
+    // OSD plane) always land above the punch layer in drawing order.
+    val video = PlayerSurfaceHost.createVideoSurface(activity, callback)
+    container.addView(video)
+    assertTrue(container.indexOfChild(video) > container.indexOfChild(punch))
+  }
 }