fix: GUI VST native window qua bridge + audio path bridge (SF2 preview/play câm)

- open_vst_gui: bo WebviewWindowBuilder/thread, chi push_control type=4 (hwnd=0 -> bridge tao window)
- main.cpp: create_native_vst_window (class SonicForge_Native_VST3_Class, 800x600, khong TOPMOST),
  tao trong ChannelWorker job, map guiWindows, capture arg2 by value (fix dangling)
- app.jsx: guard isBridgeActive() 8 cho -> bridge active thi moi note di router -> pushEvent -> bridge
  (truoc day SF2 cam vi HAS_PYFLUIDSYNTH=FALSE -> /soundfont-render 501; VST3 path cu dung nativeSf/Carla)
- E2E: SF2 NOTE_ON qua dispatchMidiEvent -> SHM peak 0.029745; Nexus GUI native hwnd OK (license/preset)
- docs: TASKS.md + TEST_NOTES.md ghi batch fix + ket qua; gitignore vendor/junk

native_bridge/include/Vst3Instrument.h

@@ -29,10 +29,8 @@ public:
     void processAudioBlock(float* outputL, float* outputR, uint32_t numSamples) override;
 
 private:
-    void* module_;      // Steinberg::IPluginFactory* (owned by module)
-    void* processor_;   // Steinberg::Vst::IComponent*
-    void* controller_;  // Steinberg::Vst::IEditController*
-    void* view_;        // Steinberg::IPlugView*
+    // pimpl: all Steinberg SDK types live in Vst3Instrument.cpp (Vst3HostState).
+    void* state_;
     std::string path_;
     double sampleRate_;
     uint32_t maxBlockSize_;