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/main.cpp

@@ -646,6 +646,14 @@ int main(int argc, char* argv[]) {
     }
     if (!fxGuiJob.empty()) return run_open_fx_gui(fxGuiJob);
 
+    // --fx-gui <job.json>: embedded GUI server (frame capture + input relay).
+    // See PLAN_MASTERBUS_VST_GUI_EMBED.md Phase 0.
+    std::string fxGuiServerJob;
+    for (int i = 1; i + 1 < argc; ++i) {
+        if (std::strcmp(argv[i], "--fx-gui") == 0) fxGuiServerJob = argv[i + 1];
+    }
+    if (!fxGuiServerJob.empty()) return run_fx_gui_server(fxGuiServerJob);
+
     // 1. Shared memory name: argv --shm <name> | env SF_SHM_NAME | default
     std::string shmName = "SonicForge_DAW_IPC";
     for (int i = 1; i + 1 < argc; ++i) {