Add tvOS display frame rate and dynamic range matching

Lets the Apple TV switch its HDMI output to match the playing video's
frame rate and dynamic range via AVDisplayManager.preferredDisplayCriteria,
driven from MPV's container-fps and video-params/gamma. Two opt-in toggles
(default off) live under Playback → Display on tvOS; both are no-ops on
other platforms. Anchor an AVKit class symbol so the linker keeps AVKit
linked — Swift only autolinks AVFoundation here, and without AVKit the
UIWindow.avDisplayManager category isn't loaded at runtime.

Yattee/Services/Player/MPVBackend.swift

@@ -133,6 +133,9 @@ final class MPVBackend: PlayerBackend {
     private var videoCodec: String = ""
     private var hwdecCurrent: String = ""
     private var hwdecInterop: String = ""
+    #if os(tvOS)
+    private var videoGamma: String = ""
+    #endif
     
     // Async initialization tracking
     private var setupTask: Task<Void, Error>?
@@ -753,6 +756,10 @@ final class MPVBackend: PlayerBackend {
         duration = 0
         bufferedTime = 0
         currentStream = nil
+        #if os(tvOS)
+        videoGamma = ""
+        clearTVDisplayCriteria()
+        #endif
         delegate?.backend(self, didChangeState: .idle)
     }
 
@@ -1631,8 +1638,19 @@ extension MPVBackend: MPVClientDelegate {
             if let fps = value as? Double, fps > 0 {
                 containerFps = fps
                 updateRenderViewFPS()
+                #if os(tvOS)
+                applyTVDisplayCriteria()
+                #endif
             }
 
+        #if os(tvOS)
+        case "video-params/gamma":
+            if let gamma = value as? String {
+                videoGamma = gamma
+                applyTVDisplayCriteria()
+            }
+        #endif
+
         case "paused-for-cache":
             if let isPausedForCache = value as? Bool {
                 pausedForCache = isPausedForCache
@@ -1706,6 +1724,25 @@ extension MPVBackend: MPVClientDelegate {
         checkAndMarkReadyIfVideoAvailable()
     }
 
+    #if os(tvOS)
+    /// Apply tvOS display-mode criteria (frame rate / dynamic range) based on the
+    /// currently-cached MPV video parameters. Safe to call repeatedly as more info
+    /// arrives (fps may land before gamma or vice versa).
+    private func applyTVDisplayCriteria() {
+        let fps = containerFps > 0 ? containerFps : nil
+        let gamma = videoGamma.isEmpty ? nil : videoGamma
+        Task { @MainActor in
+            TVDisplayModeManager.shared.apply(fps: fps, gamma: gamma)
+        }
+    }
+
+    private func clearTVDisplayCriteria() {
+        Task { @MainActor in
+            TVDisplayModeManager.shared.clear()
+        }
+    }
+    #endif
+
     /// Update render view's video FPS for display link frame rate matching
     private func updateRenderViewFPS() {
         // Use cached container-fps (set via property observation to avoid sync fetch on main thread)
@@ -1771,6 +1808,7 @@ extension MPVBackend: MPVClientDelegate {
             let hwdec = hwdecCurrent.isEmpty ? "none" : hwdecCurrent
             let interop = hwdecInterop.isEmpty ? "none" : hwdecInterop
             LoggingService.shared.debug("MPV: Video codec: \(codec), hwdec-current: \(hwdec), hwdec-interop: \(interop)", category: .mpv)
+            applyTVDisplayCriteria()
             #endif
             #if os(iOS) || os(tvOS)
             reactivateAudioSession()