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/NativeInstrumentEngine.h

@@ -7,6 +7,7 @@
 #include <sfizz.hpp>
 
 #include <map>
+#include <mutex>
 #include <memory>
 #include <string>
 #include <vector>
@@ -53,7 +54,7 @@ public:
     void processAudioBlock(float* outputL, float* outputR, uint32_t numSamples) override;
 
 private:
-    sfizz::Synth sfizzSynth;
+    sfz::Sfizz sfizzSynth;
 };
 
 // Multi-instrument host (A10): one engine instance per MIDI channel, so two
@@ -74,11 +75,12 @@ public:
     // Zero L/R then sum each assigned instrument into it.
     void renderAll(float* outputL, float* outputR, uint32_t numSamples);
 
-    size_t count() const { return channels_.size(); }
+    size_t count() const { std::lock_guard<std::mutex> lock(mu_); return channels_.size(); }
 
 private:
     static std::unique_ptr<INativeInstrument> create_instrument(InstrumentType type);
 
+    mutable std::mutex mu_;
     std::map<uint32_t, std::unique_ptr<INativeInstrument>> channels_;
     // Per-instrument scratch so engines that overwrite (not mix) stay additive.
     std::vector<float> scratchL_, scratchR_;

native_bridge/include/SharedMemoryIPC.h

@@ -2,6 +2,10 @@
 #pragma once
 #include <cstdint>
 
+#ifdef _WIN32
+#include <windows.h>
+#endif
+
 #define AUDIO_BLOCK_SIZE 256
 
 // WARNING: volatile is NOT a sync primitive. Real impl should use an

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_;