feat: beat annotator

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

@@ -0,0 +1,23 @@
+package at.lockstep.player.playback
+
+/**
+ * Fired when annotation should flush beats for the track being left:
+ * user skipped, auto-advance after a track ended, or the last track finished.
+ */
+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. */
+    val queueIndex: Int,
+    val queueSize: Int,
+    val reason: TrackBoundaryReason,
+)
+
+enum class TrackBoundaryReason {
+    /** Left this track for another (skip, previous, or middle track ended). */
+    ADVANCED_TO_OTHER_TRACK,
+
+    /** Playback of the last track in the playlist completed. */
+    LAST_TRACK_FINISHED,
+}