fix: per-track instrument + ghost note visibility

soundfontPlayer.js:
- selectInstrument: respect passed channel (don't allocate new one)
- _playNoteFluid: only use _engineChMap when channel is undefined

ghostNoteExtractor.js:
- Include ALL notes from overlapping items (not clipped)
- relative_start_beat can be negative (notes before window)
- Keep original duration instead of clamping

app/static/js/services/ghostNoteExtractor.js

@@ -40,17 +40,13 @@
           var noteAbsStart = itemStartBeat + (note.start_beat || 0);
           var noteAbsEnd = noteAbsStart + (note.duration_beats || 1);
 
-          if (noteAbsStart >= windowEndBeat || noteAbsEnd < windowStartBeat) continue;
-
-          var clampedStart = Math.max(noteAbsStart, windowStartBeat);
-          var clampedEnd = Math.min(noteAbsEnd, windowEndBeat);
-          var clampedDur = clampedEnd - clampedStart;
+          if (noteAbsStart >= windowEndBeat) continue;
 
           trackGhostNotes.push({
             id: 'ghost_' + (note.id || Math.random().toString(36).substr(2, 9)),
             pitch: note.pitch,
-            relative_start_beat: clampedStart - windowStartBeat,
-            duration_beats: clampedDur,
+            relative_start_beat: noteAbsStart - windowStartBeat,
+            duration_beats: (note.duration_beats || 1),
             velocity: note.velocity,
             original_track_name: track.name,
             original_track_color: track.color || '#888888'