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/static/js/services/nativeBridgeService.js

@@ -169,6 +169,18 @@
       return true;
     },
 
+    /**
+     * 6. REALTIME MASTER FX CHAIN (T2.4)
+     * chainJson: JSON array [{path, bypass}, ...] applied to the live master
+     * mix via the bridge control queue (type 6). Async — never blocks UI.
+     */
+    setFxChain: function (chainJson) {
+      if (!this._tauri()) return false;
+      window.__TAURI__.core.invoke('set_fx_chain', { chainJson: chainJson })
+        .catch(function (e) { console.warn('[BridgeService] set_fx_chain:', e); });
+      return true;
+    },
+
     /**
      * 5. AUDIO SINK: register consumer of PCM frames from the bridge.
      * cb(l: Float32Array, r: Float32Array)