fix: SF2 bank select, midi multi-key cut, VSTi reopen hang, multi-track stuck

- NativeInstrumentEngine: track GM bank per channel (CC0/CC32), use bank in programChange
- app.jsx: send CC0/CC32+PROGRAM before notes via __ensureBridgeProgram, dedupe, clear dedupe after async LOAD
- audioRoutingEngine/bridgeAudioNode: idempotent connect (no disconnect-flush on re-connect), fixes note cut & multi-track stuck
- main.cpp: remove 10s poll in OPEN_GUI control job (blocked realtime loop, watchdog race), VstWindowProc stores channel not inst pointer, cleanup gui maps on WM_DESTROY

app/static/js/services/audioRoutingEngine.js

@@ -6,13 +6,13 @@
   var engine = {
     _connected: false,
     _trackId: null,
+    _dest: null,
 
     isConnected: function () { return this._connected; },
 
     /** bridgeNode = window.BridgeAudioNode; trackCtx = track node ({ sfEntry, gainNode }). */
     connect: function (bridgeNode, trackCtx, trackId) {
       if (!bridgeNode || !bridgeNode.getOutputNode) return false;
-      this.disconnect();
       var dest = null;
       if (trackCtx && trackCtx.sfEntry) dest = trackCtx.sfEntry;
       else if (trackCtx && trackCtx.gainNode) dest = trackCtx.gainNode;
@@ -21,10 +21,16 @@
         console.warn('[AudioRoutingEngine] no destination (no trackCtx / masterBus)');
         return false;
       }
+      // Idempotent: updateSfRouting/ensureMasteringRouting chay tren MOI keydown/
+      // note-on — neu da noi dung dest thi KHONG disconnect (disconnect flush ring
+      // -> am phim dang vang bi cat). Chi doi khi dest/track thuc su khac.
+      if (this._connected && this._dest === dest && this._trackId === (trackId || null)) return true;
+      this.disconnect();
       if (!bridgeNode.isReady()) bridgeNode.init();
       bridgeNode.connect(dest);
       this._connected = true;
       this._trackId = trackId || null;
+      this._dest = dest;
       console.log('[AudioRoutingEngine] bridge audio -> ' + (trackId || 'masterBus'));
       return true;
     },
@@ -33,6 +39,7 @@
       if (window.BridgeAudioNode) window.BridgeAudioNode.disconnect();
       this._connected = false;
       this._trackId = null;
+      this._dest = null;
     }
   };