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/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();
 };