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;