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/include/Vst3Instrument.h

@@ -27,6 +27,7 @@ public:
     bool openGUI(void* parentWindowHandle) override;
     bool reloadForGUI() override;
     bool attachView(void* parentWindowHandle) override;
+    bool hasAttachedView() const override;
     void closeGUI() override;
     void setChannel(uint32_t ch) { channel_ = ch; }
     bool needsReload() const override { return hasAttachedOnce_ && !guiAttached_; }