fix: indexing of beat annotations after unpaired tracks

app/src/main/java/at/lockstep/player/playback/TrackBoundaryEvent.kt

@@ -8,8 +8,10 @@ data class TrackBoundaryEvent(
     val trackId: String,
     val title: String,
     val artist: String,
-    /** 0-based index in the current play queue when this track was current. */
+    /** 0-based index in the paired-only play queue when this track was current. */
     val queueIndex: Int,
+    /** 0-based position in the full Spotify playlist (used for beat/run-data file slots). */
+    val playlistPosition: Int,
     val queueSize: Int,
     val reason: TrackBoundaryReason,
 )