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/MixerApi.cpp

@@ -285,6 +285,10 @@ std::string MixerApi::getStateJson() const
         writer.write_member("label", cs.label);
         writer.write_member("hpEnabled", cs.hpEnabled);
         writer.write_member("hpFrequency", (double)cs.hpFrequency);
+        writer.write_member("vuLeft", (double)cs.vuLeft);
+        writer.write_member("vuRight", (double)cs.vuRight);
+        writer.write_member("gateOpen", cs.gateOpen);
+        writer.write_member("compressorReduction", (double)cs.compressorReduction);
         writer.end_object();
     }
     writer.write_raw("]");
@@ -309,6 +313,8 @@ std::string MixerApi::getStateJson() const
         writer.write_member("type", typeStr);
         writer.write_member("volume", (double)bs.volume);
         writer.write_member("mute", bs.mute);
+        writer.write_member("vuLeft", (double)bs.vuLeft);
+        writer.write_member("vuRight", (double)bs.vuRight);
         writer.end_object();
     }
     writer.write_raw("]");
@@ -346,6 +352,10 @@ std::string MixerApi::getStateJson() const
     }
     writer.write_raw("]");
 
+    // Physical I/O info
+    writer.write_member("physicalInputCount", (int64_t)mixerEngine_->physicalInputCount());
+    writer.write_member("physicalOutputCount", (int64_t)mixerEngine_->physicalOutputCount());
+
     writer.end_object();
     return ss.str();
 }

src/MixerEngine.cpp

@@ -499,6 +499,15 @@ MixerEngine::MixerSnapshot MixerEngine::captureSnapshot() const
         cs.label = channel->label();
         cs.hpEnabled = channel->hpEnabled();
         cs.hpFrequency = channel->hpFrequency();
+        cs.vuLeft = channel->vuLeft();
+        cs.vuRight = channel->vuRight();
+
+        // Gate detection: gate is "open" when signal exceeds approx -50dB threshold
+        // (mapped from actual gate state once a gate module is implemented)
+        cs.gateOpen = (cs.vuLeft > -50.0f || cs.vuRight > -50.0f);
+
+        // Compressor gain reduction (placeholder — 0 dB when no compressor active)
+        cs.compressorReduction = 0.0f;
 
         for (size_t i = 0; i < channel->auxSendCount(); ++i) {
             cs.auxSendLevels.push_back(channel->auxSend(i).level);
@@ -514,6 +523,8 @@ MixerEngine::MixerSnapshot MixerEngine::captureSnapshot() const
         bs.type = bus->type();
         bs.volume = bus->volume();
         bs.mute = bus->mute();
+        bs.vuLeft = bus->vuLeft();
+        bs.vuRight = bus->vuRight();
         snap.buses.push_back(bs);
     }
 
@@ -570,8 +581,12 @@ std::vector<MixerEngine::MixerOutputRoute> MixerEngine::findOutputRoutesForBus(i
 
 // --- Auto channel creation ---
 
-void MixerEngine::autoCreateChannels(uint32_t inputChannelCount)
+void MixerEngine::autoCreateChannels(uint32_t inputChannelCount, uint32_t outputChannelCount)
 {
+    // Store the physical I/O channel counts for later queries
+    physicalInputCount_ = inputChannelCount;
+    physicalOutputCount_ = outputChannelCount;
+
     // Remove existing channels if any
     for (int i = (int)channels_.size() - 1; i >= 0; --i) {
         removeChannel(i);
@@ -589,24 +604,18 @@ void MixerEngine::autoCreateChannels(uint32_t inputChannelCount)
     }
 
     // Set default output routes:
-    // For 1-2 channels: master bus → physical 1-2 (backward compatible)
+    // Always route master bus to physical 1-2 (the main monitor output)
-    // For 3+ channels: master bus → physical 1-2, then pair remaining channels
+    // For multi-channel setups, individual channel routing is configured via the output routing UI.
-    //   in groups of 2 to physical 3-4, 5-6, etc.
     outputRoutes_.clear();
 
     if (!masterBus_) return;
 
-    // Always route master bus to physical 1-2
     MixerOutputRoute masterRoute;
     masterRoute.sourceBusId = masterBus_->id();
     masterRoute.sourceStartChannel = 0;
     masterRoute.targetStartChannel = 0;
     masterRoute.channels = 2;
     outputRoutes_.push_back(masterRoute);
-
-    // If more than 2 channels, don't auto-create additional output routes.
-    // The user should configure them via the output routing UI.
-    // (Aux buses, subgroups, etc. are not auto-routed)
 }
 
 void MixerEngine::applySnapshot(const MixerSnapshot& snapshot)

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,