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/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();