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/DrumPad.h

@@ -64,6 +64,10 @@ public:
     float sustain = 1.0f;
     float release = 0.05f;
 
+    // Per-pad filter
+    float filterCutoff = 20000.0f;  // Hz
+    float filterReso   = 0.707f;    // Q
+
     // State
     bool isPlaying() const { return playing; }
     juce::String getLoadedFileName() const { return loadedFileName; }
@@ -87,6 +91,10 @@ private:
     bool playing = false;
     float currentVelocity = 1.0f;
 
+    // Per-pad low-pass filter (stereo)
+    juce::dsp::IIR::Filter<float> filterL, filterR;
+    float lastCutoff = 20000.0f;
+
     // ADSR state
     enum class EnvelopeStage { Idle, Attack, Decay, Sustain, Release };
     EnvelopeStage envStage = EnvelopeStage::Idle;