Add FX chain: per-pad filter, master compressor/EQ/distortion/reverb, VU meter

- Per-pad low-pass filter (cutoff + resonance) applied in DrumPad render
- Master bus FX chain in PluginProcessor::applyMasterFx():
  - Distortion (tanh waveshaper with drive/mix)
  - 3-band EQ (low shelf 200Hz, peak 1kHz, high shelf 5kHz)
  - Compressor (juce::dsp::Compressor with threshold/ratio)
  - Reverb (juce::dsp::Reverb with size/decay)
- Master volume + pan with constant-power pan law
- VU meter: RMS level measurement with exponential smoothing
- All FX panel and master panel knobs now connected to audio processing
- SampleEditorPanel cutoff/reso knobs linked to DrumPad filter params

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Source/PluginProcessor.h

@@ -40,7 +40,6 @@ public:
 
     juce::AudioFormatManager& getFormatManager() { return formatManager; }
 
-    // Find pad by MIDI note
     DrumPad* findPadForNote (int midiNote);
 
     // Kit management
@@ -48,13 +47,42 @@ public:
     void saveKitPreset (const juce::File& file);
     void loadKitPreset (const juce::File& file);
 
+    // Master FX parameters (read by editor from FxPanel/MasterPanel)
+    std::atomic<float> masterVolume { 1.0f };
+    std::atomic<float> masterPan    { 0.0f };
+    std::atomic<float> masterTune   { 0.0f };
+
+    // FX params
+    std::atomic<float> compThreshold { -12.0f };
+    std::atomic<float> compRatio     { 4.0f };
+    std::atomic<float> eqLo  { 0.0f };
+    std::atomic<float> eqMid { 0.0f };
+    std::atomic<float> eqHi  { 0.0f };
+    std::atomic<float> distDrive { 0.0f };
+    std::atomic<float> distMix   { 0.0f };
+    std::atomic<float> reverbSize  { 0.3f };
+    std::atomic<float> reverbDecay { 0.5f };
+
+    // VU meter levels (written by audio thread, read by GUI)
+    std::atomic<float> vuLevelL { 0.0f };
+    std::atomic<float> vuLevelR { 0.0f };
+
 private:
     std::array<DrumPad, maxPads> pads;
     int numActivePads = defaultNumPads;
     juce::AudioFormatManager formatManager;
 
-    // Default MIDI mapping (GM drum map)
+    // Master FX chain
+    juce::dsp::Reverb reverb;
+    juce::dsp::Compressor<float> compressor;
+    juce::dsp::IIR::Filter<float> eqLoFilterL, eqLoFilterR;
+    juce::dsp::IIR::Filter<float> eqMidFilterL, eqMidFilterR;
+    juce::dsp::IIR::Filter<float> eqHiFilterL, eqHiFilterR;
+
+    double currentSampleRate = 44100.0;
+
     void initializeDefaults();
+    void applyMasterFx (juce::AudioBuffer<float>& buffer);
 
     JUCE_DECLARE_NON_COPYABLE_WITH_LEAK_DETECTOR (InstaDrumsProcessor)
 };