feat: embed native VST GUI in mastering + fx rack panels (bridge --fx-gui frame capture + input relay)

Linux parts (0L1-4L1 per TASKS_LINUX_WINDOWS.md):
- native_bridge: FxGuiServer.cpp HTTP server (Winsock) — /frame BitBlt +
  PrintWindow fallback -> JPEG (libjpeg-turbo q70), /input PostMessage relay
  (mouse/wheel/key, coords scaled), /close, /ping; RenderFxJob fxGuiSetup +
  run_fx_gui_server (offscreen); main.cpp --fx-gui dispatch; vcpkg +
  CMakeLists WIN32-only + JPEG.
- api: open_fx_gui embed flag spawn --fx-gui, parse SF_FXGUI_PORT -> embed_url,
  fx-gui/close endpoint; 4 unit tests.
- ui: VstGuiEmbed component (frame refresh 100ms, ping 2s, input relay,
  scale coords) shared by MasteringModal + FXRackModal; swap graph<->GUI,
  cleanup on unmount, dead-bridge toast fallback.
- docs: TASKS_LINUX_WINDOWS.md, WALKTHROUGH_WINDOWS.md, PLAN status.
Windows tasks 0W1-4W1 remain (build + E2E real VST).

native_bridge/src/RenderFxJob.cpp

@@ -931,12 +931,11 @@ static void fxGuiDetachSafe(void* fx) {
 #endif
 }
 
-int run_open_fx_gui(const std::string& jobPath) {
-    // The editor window and its message pump live on this (STA) thread for the
-    // whole GUI session — VST3 editors need the COM apartment + pump together.
-    CoInitializeEx(nullptr, COINIT_APARTMENTTHREADED);
-    struct ComGuard { ~ComGuard() { CoUninitialize(); } } comGuard;
-
+// Shared setup for both GUI modes: parse job, load plugin, create host window
+// (offscreen for embedded capture), attach editor (SEH-guarded). Returns 0 ok,
+// 1 job/argument error, 2 plugin load or editor attach failure.
+static int fxGuiSetup(const std::string& jobPath, Vst3Fx* fx, HWND* hwndOut,
+                      bool offscreen, LONG_PTR userData) {
     // 1. Parse job { path, name }.
     std::ifstream jf(jobPath, std::ios::binary);
     if (!jf) { std::cerr << "[FxGui] cannot read job file: " << jobPath << std::endl; return 1; }
@@ -954,31 +953,47 @@ int run_open_fx_gui(const std::string& jobPath) {
     if (path.empty()) { std::cerr << "[FxGui] job missing path" << std::endl; return 1; }
 
     // 2. Load the plugin (fixed 44.1k/512 — GUI only, no audio processed here).
-    Vst3Fx fx;
-    if (!fx.load(path, 44100.0, 512)) { std::cerr << "[FxGui] plugin load failed: " << path << std::endl; return 2; }
+    if (!fx->load(path, 44100.0, 512)) { std::cerr << "[FxGui] plugin load failed: " << path << std::endl; return 2; }
 
-    // 3. Host window.
+    // 3. Host window. Embedded mode parks it far off-screen (never steals
+    //    focus; still visible to GDI so BitBlt captures real pixels).
     WNDCLASSA wc = {};
     wc.lpfnWndProc = FxGuiWndProc;
     wc.hInstance = GetModuleHandleA(nullptr);
     wc.lpszClassName = kFxGuiWndClass;
     RegisterClassA(&wc);
-    FxGuiCtx ctx;
+    const int x = offscreen ? -20000 : CW_USEDEFAULT;
+    const int y = offscreen ? -20000 : CW_USEDEFAULT;
     HWND hwnd = CreateWindowExA(0, kFxGuiWndClass,
                                 (name.empty() ? "VST FX" : name).c_str(),
                                 WS_OVERLAPPEDWINDOW | WS_VISIBLE | WS_CLIPCHILDREN | WS_CLIPSIBLINGS,
-                                CW_USEDEFAULT, CW_USEDEFAULT, 800, 600,
+                                x, y, 800, 600,
                                 nullptr, nullptr, GetModuleHandleA(nullptr), nullptr);
     if (!hwnd) { std::cerr << "[FxGui] create window failed" << std::endl; return 1; }
-    SetWindowLongPtrA(hwnd, GWLP_USERDATA, (LONG_PTR)&ctx);
+    SetWindowLongPtrA(hwnd, GWLP_USERDATA, userData);
 
     // 4. Attach the plugin editor (SEH-guarded — plugin GUI code may crash).
-    if (!fxGuiAttachSafe(&fx, hwnd)) {
+    if (!fxGuiAttachSafe(fx, hwnd)) {
         std::cerr << "[FxGui] editor attach failed for " << path << std::endl;
         DestroyWindow(hwnd);
         return 2;
     }
     std::cout << "[FxGui] editor open: " << (name.empty() ? path : name) << std::endl;
+    *hwndOut = hwnd;
+    return 0;
+}
+
+int run_open_fx_gui(const std::string& jobPath) {
+    // The editor window and its message pump live on this (STA) thread for the
+    // whole GUI session — VST3 editors need the COM apartment + pump together.
+    CoInitializeEx(nullptr, COINIT_APARTMENTTHREADED);
+    struct ComGuard { ~ComGuard() { CoUninitialize(); } } comGuard;
+
+    Vst3Fx fx;
+    HWND hwnd = nullptr;
+    FxGuiCtx ctx;
+    const int rc = fxGuiSetup(jobPath, &fx, &hwnd, /*offscreen=*/false, (LONG_PTR)&ctx);
+    if (rc) return rc;
 
     // 5. Pump messages until the window is destroyed.
     MSG msg;
@@ -995,6 +1010,25 @@ int run_open_fx_gui(const std::string& jobPath) {
     return 0;
 }
 
+// Embedded GUI server mode: same setup but off-screen, then hand the message
+// pump + HTTP capture/input to FxGuiServer.cpp (see PLAN_MASTERBUS_VST_GUI_EMBED.md).
+extern int fxGuiServerLoop(HWND hwnd);
+
+int run_fx_gui_server(const std::string& jobPath) {
+    CoInitializeEx(nullptr, COINIT_APARTMENTTHREADED);
+    struct ComGuard { ~ComGuard() { CoUninitialize(); } } comGuard;
+
+    Vst3Fx fx;
+    HWND hwnd = nullptr;
+    const int rc = fxGuiSetup(jobPath, &fx, &hwnd, /*offscreen=*/true, 0);
+    if (rc) return rc;
+
+    const int srv = fxGuiServerLoop(hwnd);
+    fxGuiDetachSafe(&fx);
+    std::cout << "[FxGuiServer] closed" << std::endl;
+    return srv;
+}
+
 #else  // !_WIN32
 
 int run_open_fx_gui(const std::string& jobPath) {
@@ -1003,6 +1037,12 @@ int run_open_fx_gui(const std::string& jobPath) {
     return 1;
 }
 
+int run_fx_gui_server(const std::string& jobPath) {
+    (void)jobPath;
+    std::cerr << "[FxGui] GUI mode chỉ hỗ trợ Windows" << std::endl;
+    return 1;
+}
+
 #endif  // _WIN32
 
 // --- --scan: classify VST3 modules as instrument/effect ----------------------