Phase 0 (PLAN_MASTERBUS_FX_RACK_VST): banner ra stderr de stdout sach cho --scan JSON / --fx-gui port

Test /c/tmp/test_fx_phase0.py: 7/7 pass — gain0 bit-exact, gain -6dB RMS/2,
normalize peak<=0.25, empty chain rc=1, Ozone 11 Maximizer VST3 render khac
input (88172 samples), --scan phan loai dung (Ozone is_fx, Scaler 2 instrument).

native_bridge/src/main.cpp

@@ -721,7 +721,9 @@ int main(int argc, char* argv[]) {
     // fx-gui port reader) survives FreeConsole.
     if (!std::getenv("SF_KEEP_CONSOLE")) FreeConsole();
 #endif
-    std::cout << "[NativeBridge] Starting DAW Host Bridge Engine..." << std::endl;
+    // stderr, not stdout: stdout is reserved for machine-readable output in
+    // one-shot modes (--scan JSON, --fx-gui "SF_FXGUI_PORT=...").
+    std::cerr << "[NativeBridge] Starting DAW Host Bridge Engine..." << std::endl;
 #ifdef _WIN32
     // Disable Power Throttling (Efficiency Mode) for this background process
 #ifndef PROCESS_POWER_THROTTLING_IGNORE_TIMER_RESOLUTION