feat: auto-loop on range selection + per-track instrument isolation

- isLooping defaults to true on subTab creation (auto-loop when
  range selection is set)
- Per-track instrument confirmed: selectInstrument respects passed
  channel, _playNoteFluid bypasses _engineChMap when channel is
  explicit, no-instrument fallback sets program 0 (default piano)
  instead of inheriting another track's program

app/static/js/app.jsx

@@ -8296,6 +8296,7 @@ const App = () => {
       instrumentName: track.instrumentName,
       currentTime: 0,
       isPlaying: false,
+      isLooping: true,
       selectionStart: null,
       selectionEnd: null,
       viewport_start_bar: 0.0,