feat: native host bridge integration — C++ bridge, Rust SHM, JS routing, build scripts, docs

- native_bridge/: InstrumentEngineManager multi-channel, sample-accurate, CC/program/pitchbend, transport, Vst3Instrument stub (HAVE_VST3SDK)
- src-tauri: shm.rs, bridge spawn + audio pump + health monitor, open_vst_gui, externalBin, commands
- app: UnifiedMidiRouter, NativeBridgeService, bridgeAudioNode, audioRoutingEngine, Plugin Manager UI, Bridge/WASM indicator, set_position sync
- build: 3 ps1 (force-added, build/ ignored), verify_bundle --check-bridge, CI workflow
- docs: TASKS.md, TEST_NOTES.md (Windows verify checklist), install/report updates

native_bridge/include/Vst3Instrument.h

@@ -0,0 +1,43 @@
+// native_bridge/include/Vst3Instrument.h
+// VST3 host instrument (A7). Compiled ALWAYS; the real vst3sdk wiring is
+// inside #ifdef HAVE_VST3SDK (set by CMake when the vst3sdk submodule is
+// present). Without the SDK the class degrades to a no-op stub so the bridge
+// still builds for SF2/SF3/SFZ.
+#ifndef VST3_INSTRUMENT_H
+#define VST3_INSTRUMENT_H
+
+#include "INativeInstrument.h"
+
+#include <string>
+
+class Vst3Instrument : public INativeInstrument {
+public:
+    Vst3Instrument();
+    ~Vst3Instrument() override;
+
+    bool loadPlugin(const std::string& path, double sampleRate);
+
+    bool init(double sampleRate, uint32_t maxBlockSize) override;
+    void selectProgram(uint32_t channel, uint32_t bank, uint32_t program) override;
+    void noteOn(uint32_t channel, uint32_t pitch, float velocity, uint32_t sampleOffset) override;
+    void noteOff(uint32_t channel, uint32_t pitch, uint32_t sampleOffset) override;
+    void controlChange(uint32_t channel, uint32_t cc, uint32_t value) override;
+    void programChange(uint32_t channel, uint32_t program) override;
+    void pitchBend(uint32_t channel, uint32_t bend14) override;
+    bool openGUI(void* parentWindowHandle) override;
+    void closeGUI() override;
+    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*
+    std::string path_;
+    double sampleRate_;
+    uint32_t maxBlockSize_;
+    bool loaded_;
+    bool guiAttached_;
+};
+
+#endif // VST3_INSTRUMENT_H