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/src/main.cpp

@@ -299,6 +299,26 @@ int main(int argc, char* argv[]) {
                 uint32_t ch = c.channel & 0xF;
                 if (!workers[ch]) workers[ch] = std::make_unique<ChannelWorker>();
                 workers[ch]->post([&instruments, t, ch, path, sampleRate, block]() {
+#ifdef _WIN32
+                    // DONG cua so editor dang mo cua channel TRUOC khi assign():
+                    // thay the inst (VST3 -> SF2/inst khac) ma editor con song ->
+                    // old inst destructor goi view->removed() tren HWND con hoat
+                    // dong -> plugin block -> treo bridge. DestroyWindow chay tren
+                    // CUNG worker thread so huu window; WM_DESTROY goi closeGUI()
+                    // dung thu tu. Phai erase map TRUOC DestroyWindow (WM_DESTROY
+                    // handler lay g_guiMutex lai — khong duoc giu lock khi destroy).
+                    HWND hToDestroy = nullptr;
+                    {
+                        std::lock_guard<std::mutex> lock(g_guiMutex);
+                        auto git = g_guiWindows.find(ch);
+                        if (git != g_guiWindows.end()) {
+                            hToDestroy = (HWND)git->second;
+                            g_guiWindows.erase(git);
+                            g_hwndToCh.erase(hToDestroy);
+                        }
+                    }
+                    if (hToDestroy && IsWindow(hToDestroy)) DestroyWindow(hToDestroy);
+#endif
                     std::cerr << "[dbg] load thread start ch=" << ch
                               << " type=" << (int)t << " path=" << path << std::endl;
                     bool ok = instruments.assign(ch, t, path, sampleRate, block);