fix: store midiChannel on track object for consistent per-track playback

- setTrackInstrumentWithProgram stores midiChannel on the track
- schedulePianoRollMidi reads track.midiChannel instead of
  recomputing from index (can be inconsistent across calls)
- selectInstrument also stores midiChannel if not already set

app/static/js/app.jsx

@@ -6775,7 +6775,12 @@ const App = () => {
         soundfont_program: sfProg !== undefined ? sfProg : 0,
         soundfont_id: isSfInstrument ? instrumentId.replace('sf_', '') : ''
       } : undefined;
-      return { ...t, instrumentId, instrumentProgram: sfProg, instrumentName: displayName, soundfont_bank: sfBank, soundfont_program: sfProg, synth_engine: synthEngine, type: hasInstrument ? 'MIDI' : (t.type === 'MIDI' ? 'audio' : t.type) };
+      var mt = activeTracksRef.current || tracks;
+      var midx = 0;
+      for (var mi = 0; mi < mt.length; mi++) { if (mt[mi].id === trackId) { midx = mi; break; } }
+      var mch = sfBank === 128 ? 9 : (midx >= 9 ? midx + 1 : midx);
+      if (mch >= 16) mch = 0;
+      return { ...t, midiChannel: mch, instrumentId, instrumentProgram: sfProg, instrumentName: displayName, soundfont_bank: sfBank, soundfont_program: sfProg, synth_engine: synthEngine, type: hasInstrument ? 'MIDI' : (t.type === 'MIDI' ? 'audio' : t.type) };
     }));
     setInstrumentDropdownTrackId(null);
     setInstrumentDropdownBtnRect(null);
@@ -6790,6 +6795,10 @@ const App = () => {
       for (var i = 0; i < allTracks.length; i++) { if (allTracks[i].id === trackId) { tidx = i; break; } }
       var ch = sfBank === 128 ? 9 : (tidx >= 9 ? tidx + 1 : tidx);
       if (ch >= 16) ch = 0;
+      // Store channel for consistent per-track instrument playback
+      if (!allTracks[tidx] || allTracks[tidx].midiChannel === undefined) {
+        updateActiveTracks(prev => prev.map(t => t.id === trackId ? { ...t, midiChannel: ch } : t));
+      }
       window.SonicSF.selectInstrument(ch, sfBank || 0, sfProg || 0, sfId);
     }
     setSubTabs(prev => prev.map(s => {
@@ -10398,8 +10407,7 @@ const App = () => {
     const instrumentProgram = track ? track.instrumentProgram : undefined;
     const synthEngine = track ? track.synth_engine : undefined;
     var allTracks = activeTracksRef.current || [];
-    var mainCh = 0;
-    for (var i = 0; i < allTracks.length; i++) { if (allTracks[i].id === st.trackId) { mainCh = i >= 9 ? i + 1 : i; if (mainCh >= 16) mainCh = 0; break; } }
+    var mainCh = track ? (track.midiChannel !== undefined ? track.midiChannel : 0) : 0;
     // Compute session beat offset from target item
     var sessionBeatOffset = 0;
     var targetTrk = allTracks.find(function(t) { return t.id === st.trackId; });
@@ -10430,8 +10438,7 @@ const App = () => {
       var ghostSynth = layer.synthEngine || (ghostTrack ? ghostTrack.synth_engine : undefined);
       if (ghostProg === undefined && !ghostSynth) return;
       var ghostDest = getOrCreateTrackNode(ghostTrack, context);
-      var ghostCh = 0;
-      for (var i = 0; i < allTracks.length; i++) { if (allTracks[i].id === layer.trackId) { ghostCh = i >= 9 ? i + 1 : i; if (ghostCh >= 16) ghostCh = 0; break; } }
+      var ghostCh = ghostTrack ? (ghostTrack.midiChannel !== undefined ? ghostTrack.midiChannel : 0) : 0;
       layer.notes.forEach(function(note) {
         var beat = (note.start_beat || 0) + sessionBeatOffset;
         var dur = note.duration_beats || 1;