fix(bridge): VSTi GUI crash on instrument switch — PostMessage WM_CLOSE instead of cross-thread DestroyWindow

- LOAD job posts WM_CLOSE to window created on main thread (DestroyWindow from worker crashed 0xc000041d, muting all tracks)
- handleOpenGui + attach jobs skip when Vst3Instrument::hasAttachedView() (state_ && guiAttached_)
- attach failure posts WM_CLOSE; post_close_gui erases entry only when it points at the same hwnd
- cap openVstGuiRetry at 5 to stop frontend retry spam

native_bridge/src/Vst3Instrument.cpp

@@ -635,6 +635,10 @@ bool Vst3Instrument::attachView(void* parentWindowHandle) {
 #endif
 }
 
+bool Vst3Instrument::hasAttachedView() const {
+    return state_ != nullptr && guiAttached_;
+}
+
 bool Vst3Instrument::reload() {
 #ifndef HAVE_VST3SDK
     return false;