T15: drop WebAudio master for tracks - native master gain + track gain/pan IPC (SF/VST2/VST3 bridges), NativeAudioService + /api/v1/native router, native-first TrackInstrument, test matrix 8 passed

native_host/SFHost.cpp

@@ -409,6 +409,18 @@ __declspec (dllexport) int32 SF_FS_SetMasterLimiter (int32 handle, int32 active,
     return 0;
 }
 
+// T15: master fader — linear, ap sau limiter+clip trong mixer (mirror
+// masterBus.output.gain WebAudio).
+__declspec (dllexport) int32 SF_FS_SetMasterGain (int32 handle, float gainLinear)
+{
+    std::lock_guard<std::mutex> lock (g_mutex);
+    auto it = g_instances.find (handle);
+    if (it == g_instances.end ())
+        return -1;
+    it->second->mixer.setMasterGain (gainLinear);
+    return 0;
+}
+
 // Đóng: stop audio loop, xóa synth, giải phóng DLL.
 __declspec (dllexport) int32 SF_FS_Close (int32 handle, char* err, int32 err_cap)
 {