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/bridgeAudioNode.js

@@ -12,6 +12,7 @@
   var _gainNode = null;
   var _ctx = null;
   var _initialized = false;
+  var _dest = null; // destination gain node da noi (chong connect trung)
 
   function _getCtx() {
     if (_ctx) return _ctx;
@@ -78,6 +79,7 @@
     _spn = null;
     _gainNode = null;
     _ctx = null;
+    _dest = null;
   }
 
   window.BridgeAudioNode = {
@@ -85,7 +87,13 @@
     onAudio: onAudio,
     flush: flush,
     getOutputNode: getOutputNode,
-    connect: function (dest) { if (_gainNode) _gainNode.connect(dest); },
+    connect: function (dest) {
+      if (!_gainNode) return;
+      if (_dest === dest) return;              // da noi — khong noi trung (double-sum)
+      if (_dest) { try { _gainNode.disconnect(_dest); } catch (e) {} }
+      _gainNode.connect(dest);
+      _dest = dest;
+    },
     disconnect: disconnect,
     isReady: function () { return _initialized; }
   };