Fix black video, freezes, autoplay and clock on Apple TV HD (A8)

Fixes four bugs behind yattee#956 on Apple TV HD (AppleTV5,3, A8,
tvOS 26.5), where video played audio only with a permanently black
picture, never autoplayed, the clock stayed at 0:00, and navigating
during/after playback could freeze the whole app.

1. A8 GL driver deadlock (device-specific). glTexImage2D uploading
   float32 data into a float16 texture hangs in libGLImage's
   glgProcessPixelsWithProcessor (a dispatch_group_wait that never
   returns), permanently wedging mpv_render_context_render. mpv 0.37
   (commit 703f1588) turned on dithering (dither-depth=auto) and
   LUT-based scalers by default; on the A8 (no GL_EXT_texture_norm16)
   both upload float weights via that fatal conversion, and per-frame
   CPU plane uploads (videotoolbox-copy / software decode) hit the same
   path. Yattee 1.x shipped mpv 0.36 with these off, which is why the
   device worked there. Worked around on AppleTV5,3 only: dither-depth=no,
   hwdec=videotoolbox (zero-copy via IOSurface), and bilinear scalers.
   Confirmed by two on-device lldb backtraces.

2. Swift cooperative-pool starvation (all platforms). MPVClient's event
   loop ran as Task.detached and blocked a cooperative-pool thread for
   the client's entire lifetime. With two clients alive (active +
   pre-warmed) on a 2-core A8, both pool threads were held and every
   await in the app starved: the load pipeline froze at its first
   Task.sleep, killing autoplay, Now Playing and progress saving. Moved
   the event loop to a dedicated Thread; EAGLContext creation now bridges
   through GCD instead of Task.detached.

3. Leaked time-update gate (all platforms). PlayerService dropped all
   time updates while loadingVideoID was set; when a load task died
   mid-flight the flag leaked and the clock stayed at 0:00 during
   playback. Heal the gate on the ready/playing transition and leak-proof
   play() with a defer; log the previously silent load cancellation.

4. Orphaned player view (all platforms). Detach the shared render view in
   MPVContainerView.deinit when no successor container exists, and skip
   framebuffer recreation while the view is windowless, to stop a 100% CPU
   SwiftUI trait-update loop when opening a settings detail during playback.

Yattee/Services/Player/PlayerService.swift

@@ -250,6 +250,13 @@ final class PlayerService {
         // Mark that we're loading this video - time updates will be ignored until loading completes
         loadingVideoID = video.id
 
+        // Release the gate on any exit path (a newer play() claims it with its own
+        // id, so only clear our own). Prevents the time-update gate from leaking if
+        // the load flow exits early (issue #956).
+        defer {
+            if loadingVideoID == video.id { loadingVideoID = nil }
+        }
+
         state.setPlaybackState(.loading)
         state.isFirstFrameReady = false  // Reset until first frame of new video is rendered
         state.isBufferReady = false  // Reset until buffer is ready for smooth playback
@@ -2844,6 +2851,14 @@ extension PlayerService: PlayerBackendDelegate {
 
     func backend(_ backend: any PlayerBackend, didChangeState playbackState: PlaybackState) {
         LoggingService.shared.debug("Backend state changed to: \(playbackState)", category: .player)
+
+        // Clear the time-update gate once playback is ready/playing, in case the
+        // load flow exited before doing so (issue #956). Safe: stale updates from a
+        // previous video can only arrive before the new video's ready transition.
+        if playbackState == .ready || playbackState == .playing, loadingVideoID != nil {
+            loadingVideoID = nil
+        }
+
         state.setPlaybackState(playbackState)
         delegate?.playerService(self, didChangeState: playbackState)
     }