fix: VST3 teardown under lock + re-attach hang + editor window on replace

- InstrumentEngineManager::assign: destroy replaced instrument OUTSIDE mu_
  (VST3 terminate/removed can block -> audio loop stall -> bridge Not
  Responding, transport stop hangs, notes never off)
- main.cpp LOAD job: DestroyWindow any open editor window of the channel
  before assign (replace VSTi -> SF2 with editor alive hung in removed())
- Vst3Instrument: reload() fresh plugin instance on second openGUI after
  close (Nexus etc. hang at view->attached() twice on same component)

native_bridge/include/Vst3Instrument.h

@@ -36,6 +36,8 @@ private:
     uint32_t maxBlockSize_;
     bool loaded_;
     bool guiAttached_;
+    bool hasAttachedOnce_;
+    bool reload();
 };
 
 #endif // VST3_INSTRUMENT_H