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/src/Vst3Instrument.cpp

@@ -170,7 +170,8 @@ Vst3Instrument::Vst3Instrument()
       maxBlockSize_(256),
       loaded_(false),
       guiAttached_(false),
-      hasAttachedOnce_(false) {}
+      hasAttachedOnce_(false),
+      reloading_(false) {}
 
 Vst3Instrument::~Vst3Instrument() {
 #ifdef HAVE_VST3SDK
@@ -598,6 +599,11 @@ void Vst3Instrument::processAudioBlock(float* outputL, float* outputR, uint32_t
 #ifndef HAVE_VST3SDK
     return;
 #else
+    // GUI reopen rebuilds the instance on the worker thread; while the
+    // teardown runs, the audio loop must not touch state_ (flag toggled under
+    // InstrumentEngineManager::mu_ by setReloading — renderAll holds the same
+    // mutex during processAudioBlock, so the check is race-free).
+    if (reloading_) return;
     auto* s = static_cast<Vst3HostState*>(state_);
     if (!s || !s->component || numSamples == 0) return;
     FUnknownPtr<IAudioProcessor> processor(s->component);