Add VU meter data, gate status, and compressor GR to mixer IPC protocol

- MixerEngine.hpp: Add vuLeft/vuRight, gateOpen, compressorReduction to
  ChannelState and vuLeft/vuRight to BusState in MixerSnapshot
- MixerEngine.cpp (captureSnapshot): Populate VU data from channel strips
  and buses; detect gate open/closed from signal level
- MixerApi.cpp (getStateJson): Serialize VU, gate, and compressor fields
  in the JSON state response

This enables the frontend MetersPage to display real-time VU meter bars,
gate open/closed indicators, and compressor gain reduction meters.

src/MixerEngine.hpp

@@ -74,7 +74,9 @@ public:
     /// Auto-create channels based on the number of detected input channels.
     /// Removes existing channels and creates one strip per input.
     /// Also sets up default output routes.
-    void autoCreateChannels(uint32_t inputChannelCount);
+    /// @param inputChannelCount Number of physical input channels detected
+    /// @param outputChannelCount Number of physical output channels detected
+    void autoCreateChannels(uint32_t inputChannelCount, uint32_t outputChannelCount = 2);
 
     /// Remove a channel by its channel index.
     void removeChannel(int channelIndex);
@@ -148,6 +150,19 @@ public:
     /// Get all current routes.
     const std::vector<MixerRouteEntry>& routes() const { return routes_; }
 
+    /// --- Physical I/O Info ---
+
+    /// Number of physical input channels detected on the audio device.
+    uint32_t physicalInputCount() const { return physicalInputCount_; }
+    /// Number of physical output channels detected on the audio device.
+    uint32_t physicalOutputCount() const { return physicalOutputCount_; }
+
+    /// Set the physical I/O channel counts (called by autoCreateChannels or externally).
+    void setPhysicalChannelCounts(uint32_t inputs, uint32_t outputs) {
+        physicalInputCount_ = inputs;
+        physicalOutputCount_ = outputs;
+    }
+
     /// --- Audio Processing (real-time thread) ---
 
     /// Prepare all channels and allocate buffers.
@@ -199,6 +214,10 @@ public:
             bool hpEnabled;
             float hpFrequency;
             std::vector<float> auxSendLevels; // indexed by aux bus index
+            float vuLeft = -96.0f;            // dB — peak VU level
+            float vuRight = -96.0f;           // dB — peak VU level
+            bool gateOpen = true;              // gate state (true = signal passing)
+            float compressorReduction = 0.0f;  // dB of gain reduction (0 = no reduction)
         };
         struct BusState {
             int64_t id;
@@ -206,6 +225,8 @@ public:
             MixerBusType type;
             float volume;
             bool mute;
+            float vuLeft = -96.0f;   // dB — peak VU level
+            float vuRight = -96.0f;  // dB — peak VU level
         };
         std::vector<ChannelState> channels;
         std::vector<BusState> buses;
@@ -243,6 +264,10 @@ private:
     // Allocated once at prepare time
     std::vector<std::vector<float>> channelOutputBuffers_;
 
+    // Physical I/O channel counts (from audio device auto-detection)
+    uint32_t physicalInputCount_ = 0;
+    uint32_t physicalOutputCount_ = 0;
+
     // Internal helper: accumulate a channel's output to all its routed buses
     void routeChannelOutput(
         MixerChannelStrip* channel,