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/include/RenderFxJob.h

@@ -44,4 +44,13 @@ int run_scan_dir(const std::string& dirPath);
 // editor attach failure.
 int run_open_fx_gui(const std::string& jobPath);
 
+// Embedded GUI server mode (Phase 0 of PLAN_MASTERBUS_VST_GUI_EMBED.md):
+// `daw_vst_bridge --fx-gui <job.json>` — same plugin load + editor attach as
+// --open-fx-gui but the host window is parked off-screen and a tiny HTTP
+// server (FxGuiServer.cpp) serves JPEG frames (GET /frame) and relays pointer/
+// key input (POST /input) so the web UI can embed the native GUI in a panel.
+// Prints `SF_FXGUI_PORT=<port>` to stdout. Windows only. Exit codes same as
+// --open-fx-gui.
+int run_fx_gui_server(const std::string& jobPath);
+
 #endif // RENDER_FX_JOB_H