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/NativeMixer.h

@@ -38,6 +38,10 @@ public:
     // thresholdDb clamp -24..0, NaN -> -1 (mirror _clamp trong mastering_engine).
     void setLimiter (bool active, float thresholdDb);
 
+    // Master fader (T15) — mirror WebAudio masterBus.output.gain (sau mastering
+    // chain + clip; linear, khong re-clip). Mac dinh 1.0.
+    void setMasterGain (float gainLinear) { m_masterGain = gainLinear; }
+
     // Ap track gain/pan vao buffer roi (neu active) limiter + hard clip [-1,1]
     // ngay tai buffer (in-place). frames > 0.
     void processInPlace (float* outL, float* outR, int32 frames) const;
@@ -49,6 +53,7 @@ private:
     bool m_limActive = false;
     float m_limK = 1.0f;   // 1/max(0.02, t_lin)
     float m_limTk = 1.0f;  // tanh(k)
+    float m_masterGain = 1.0f; // T15: master fader (sau limiter+clip, khong re-clip)
 };
 
 } // namespace sonicforge