feat: implement md/41_INSTRUMENT.md spec

- PianoRollTabService: getParentTrackByItemId, buildActiveScope,
  getTrackMidiChannel helpers
- parent_track_id on all MIDI items for reverse track lookup
- handleSwitchMidiItem uses active_scope pattern
- Toolbar shows parent track context labels
- Channel assignment simplified to trackIndex % 16
- All playNote calls use unified getTrackMidiChannel

app/templates/index.html

@@ -18,6 +18,7 @@
     <script src="/static/js/services/soundfontPlayer.js?v=202607271245"></script>
     <script src="/static/js/services/aiGateway.js?v=202607271016"></script>
     <script src="/static/js/services/dawCommandDispatcher.js?v=202607271016"></script>
+    <script src="/static/js/services/pianoRollTabService.js?v=202607272044"></script>
     <script src="/static/js/services/ghostNoteExtractor.js?v=202607271727"></script>
     <script src="/static/js/app.precompiled.js?v=202607271245" defer></script>
     <link rel="stylesheet" href="/static/css/styles.css?v=202607271016">