feat(player): add IQueueService orchestrating album playback above the single-slot player (P11 11.F)

Queue owns ordered tracks, current index, skip-fwd/back, and auto-advance via the player's TrackEnded hook; binds through Attach (no ctor growth, no service-locator). Player-bar skip controls; empty-queue play unchanged. Adds QueueService unit tests.

DeepDrftPublic.Client/Services/AudioPlayerService.cs

@@ -43,6 +43,9 @@ public abstract class AudioPlayerService : IPlayerService, IAsyncDisposable
     /// <inheritdoc />
     public event Action? StateChanged;
 
+    /// <inheritdoc />
+    public event Action? TrackEnded;
+
     protected AudioPlayerService(AudioInteropService audioInterop, TrackMediaClient trackMediaClient)
     {
         _audioInterop = audioInterop;
@@ -268,6 +271,12 @@ public abstract class AudioPlayerService : IPlayerService, IAsyncDisposable
         CurrentTime = 0;
         Duration = null;
         await NotifyStateChanged();
+
+        // Fire AFTER the state notification so any queue orchestrator that advances on this
+        // signal selects the next track against a fully-settled idle state. Raised only on
+        // organic end-of-stream — stop/unload/track-switch go through ResetToIdle, which does
+        // not raise this — so a subscriber can treat it unambiguously as "advance the queue."
+        TrackEnded?.Invoke();
     }