feat(native): realtime master FX chain + fx-gui fast start, hide/show, non-topmost

- FxGuiServer: HWND_TOP (no longer always-on-top), /hide + /show endpoints,
  onStarted callback prints SF_FXGUI_PORT before plugin load (fast connect)
- RenderFxJob: RealtimeFxChain (mutex+worker, SEH-guarded, bypass slots,
  deferred Vst3Fx dtor); --fx-gui split into parse/create/load-attach stages
- main: FreeConsole unless SF_KEEP_CONSOLE; control type 6 SET_FX_CHAIN feeds
  g_fxChain on the audio loop; shutdown before CoUninitialize
- plugins.py: CREATE_NO_WINDOW on both Popen paths
- lib.rs: set_fx_chain command -> push_control(6)
- app.jsx/nativeBridgeService: setFxChain sync from MasteringModal; panel close
  uses /hide + refs to fix closure bug; VstGuiEmbed rect refresh -> /show

app/api/v1/plugins.py

@@ -730,7 +730,8 @@ async def open_fx_gui(req: FxGuiRequest, current_user: dict = Depends(get_curren
         if req.embed:
             proc = subprocess.Popen(cmd, stdout=subprocess.PIPE,
                                     stderr=subprocess.STDOUT, text=True,
-                                    close_fds=os.name != "nt")
+                                    close_fds=os.name != "nt",
+                                    creationflags=subprocess.CREATE_NO_WINDOW if os.name == "nt" else 0)
             # Bridge prints SF_FXGUI_PORT=<port> after binding its HTTP server.
             embed_url = None
             try:
@@ -754,7 +755,8 @@ async def open_fx_gui(req: FxGuiRequest, current_user: dict = Depends(get_curren
             _register_fx_gui_process(proc)
             return {"success": True, "started": True, "already_running": False,
                     "embed_url": embed_url, "cmd": cmd}
-        proc = subprocess.Popen(cmd, close_fds=os.name != "nt")
+        proc = subprocess.Popen(cmd, close_fds=os.name != "nt",
+                                creationflags=subprocess.CREATE_NO_WINDOW if os.name == "nt" else 0)
         proc._sf_plugin_path = plugin_path
         _register_fx_gui_process(proc)
         return {"success": True, "started": True, "already_running": False, "cmd": cmd}