fix(bridge): one editor at a time + guard reload race

- Close other channels' editor windows before attaching a new one (Option B)
- needsReload()/setReloading() guard: skip processAudioBlock during
  terminate+reload teardown to avoid UAF/hang when reopening GUI
- Wait for editor registry cleanup (5s timeout) before creating window

native_bridge/include/INativeInstrument.h

@@ -34,6 +34,13 @@ public:
     // Open/close Native GUI window
     virtual bool openGUI(void* parentWindowHandle) = 0;
     virtual void closeGUI() = 0;
+    // VST3 reopen-after-close: the bridge checks before openGUI whether the
+    // instance must be rebuilt; while it is, the engine sets reloading=true
+    // and the real-time loop skips processAudioBlock (teardown must not race
+    // the audio thread — a reload terminates the VST while renderAll may be
+    // inside processAudioBlock on the same instance).
+    virtual bool needsReload() const { return false; }
+    virtual void setReloading(bool /*on*/) {}
 
     // Real-time Audio PCM Float32 rendering loop
     virtual void processAudioBlock(float* outputL, float* outputR, uint32_t numSamples) = 0;

native_bridge/include/NativeInstrumentEngine.h

@@ -72,6 +72,11 @@ public:
 
     INativeInstrument* get(uint32_t channel);
 
+    // Mark a channel as rebuilding its VST instance (GUI reopen): the
+    // real-time loop then skips processAudioBlock for that channel so VST
+    // teardown never races the audio thread.
+    void setReloading(uint32_t channel, bool on);
+
     // Flush every sounding note on every assigned channel.
     void allNotesOff();
 

native_bridge/include/Vst3Instrument.h

@@ -26,6 +26,8 @@ public:
     void pitchBend(uint32_t channel, uint32_t bend14) override;
     bool openGUI(void* parentWindowHandle) override;
     void closeGUI() override;
+    bool needsReload() const override { return hasAttachedOnce_ && !guiAttached_; }
+    void setReloading(bool on) override { reloading_ = on; }
     void processAudioBlock(float* outputL, float* outputR, uint32_t numSamples) override;
 
 private:
@@ -37,6 +39,7 @@ private:
     bool loaded_;
     bool guiAttached_;
     bool hasAttachedOnce_;
+    bool reloading_; // guarded by InstrumentEngineManager::mu_ (set via setReloading)
     bool reload();
 };