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

native_bridge/include/RenderFxJob.h

@@ -2,6 +2,8 @@
 #ifndef RENDER_FX_JOB_H
 #define RENDER_FX_JOB_H
 
+#include <cstdint>
+#include <memory>
 #include <string>
 
 // Offline FX render mode (Phase 0 of PLAN_MASTERBUS_FX_RACK_VST.md):
@@ -53,4 +55,23 @@ int run_open_fx_gui(const std::string& jobPath);
 // --open-fx-gui.
 int run_fx_gui_server(const std::string& jobPath);
 
+// Realtime FX chain (T2.4): live VST3 audio effects on the bridge master mix.
+// setChain() is async (a worker thread loads the plugins and swaps the chain);
+// process() runs ONLY on the audio thread; shutdown() joins the worker and must
+// be called before CoUninitialize. v1: master chain only (per-track deferred).
+class RealtimeFxChain {
+public:
+    RealtimeFxChain();
+    ~RealtimeFxChain();
+    // JSON: [{"path":"...","bypass":true|false},...] — chain entries replace
+    // the previous chain atomically. May be called from any thread.
+    void setChain(const std::string& json, double sampleRate, int32_t blockSize);
+    // Audio thread only: run the chain in-place over one stereo block.
+    void process(float* inL, float* inR, uint32_t n);
+    void shutdown();
+private:
+    struct Impl;
+    std::unique_ptr<Impl> impl_;
+};
+
 #endif // RENDER_FX_JOB_H