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/NativeInstrumentEngine.cpp

@@ -195,6 +195,12 @@ INativeInstrument* InstrumentEngineManager::get(uint32_t channel) {
     return it == channels_.end() ? nullptr : it->second.get();
 }
 
+void InstrumentEngineManager::setReloading(uint32_t channel, bool on) {
+    std::lock_guard<std::mutex> lock(mu_);
+    auto it = channels_.find(channel);
+    if (it != channels_.end()) it->second->setReloading(on);
+}
+
 void InstrumentEngineManager::allNotesOff() {
     std::lock_guard<std::mutex> lock(mu_);
     for (auto& [ch, inst] : channels_) {