feat(events): add the position, playback-failure and play-count events

Three events the frontend had no way to learn about:

- `PlaybackPositionChanged` carries `player.PositionInfo`, so the seek
  bar can render what the player is doing instead of counting seconds
  itself.
- `PlaybackFailed` carries the file and the reason, from both the load
  and the play path, so a track that will not play stops being a
  silent no-op.
- `TrackPlayCountChanged` carries everything needed to patch one track
  in place. `TrackMetadataChanged` means "the tags on disk were
  rewritten" and costs the frontend its entire library cache; finishing
  a track used to emit it.

An event's cost is part of its meaning, and the expensive one must not
be reused for something cheap.

frontend/src/events.ts

@@ -8,6 +8,8 @@ export const Events = {
     SeekFailed: "SeekFailed",
     VolumeChanged: "VolumeChanged",
     MuteChanged: "MuteChanged",
+    PlaybackPositionChanged: "PlaybackPositionChanged",
+    PlaybackFailed: "PlaybackFailed",
 
     // Queue events (backend → frontend push)
     QueueChanged: "QueueChanged",
@@ -49,10 +51,28 @@ export const Events = {
     LibraryRenamed: "LibraryRenamed",
     LibraryRemoved: "LibraryRemoved",
 
-    // Tag writing events
+    // Tag writing events.
+    //
+    // TrackMetadataChanged means "tags on disk were rewritten", and the
+    // frontend answers it by throwing the whole library cache away and
+    // refetching — which is correct, because a retag can change an album
+    // name, an artist, a genre, and therefore every derived collection.
+    //
+    // It must therefore not be reused for anything cheaper.  Finishing a
+    // track used to emit it, so every song cost a full refetch: ~37 MB
+    // across the IPC per track at 50 000 tracks, and the user's track
+    // selection cleared while music played (audit perf.C1/C2).  That is
+    // what TrackPlayCountChanged below exists to separate
     TrackMetadataChanged: "TrackMetadataChanged",
     BatchWriteProgress: "BatchWriteProgress",
 
+    // Play statistics events.
+    //
+    // TrackPlayCountChanged carries everything needed to patch the one
+    // track in place, precisely so no consumer has any reason to invalidate
+    // a collection: {audioFileId, filePath, playCount, lastPlayed}
+    TrackPlayCountChanged: "TrackPlayCountChanged",
+
     // Autotag apply events — emitted while an async ApplyAsync job is in flight so the review UI can render per-folder progress
     AutotagApplyStarted: "AutotagApplyStarted",
     AutotagApplyProgress: "AutotagApplyProgress",