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.cpp

@@ -3,9 +3,17 @@
 DrumPad::DrumPad() {}
 DrumPad::~DrumPad() {}
 
-void DrumPad::prepareToPlay (double sr, int /*samplesPerBlock*/)
+void DrumPad::prepareToPlay (double sr, int samplesPerBlock)
 {
     sampleRate = sr;
+
+    // Prepare per-pad filter
+    juce::dsp::ProcessSpec spec { sr, (juce::uint32) samplesPerBlock, 1 };
+    filterL.prepare (spec);
+    filterR.prepare (spec);
+    filterL.reset();
+    filterR.reset();
+    lastCutoff = filterCutoff;
 }
 
 void DrumPad::releaseResources()
@@ -339,6 +347,18 @@ void DrumPad::renderNextBlock (juce::AudioBuffer<float>& outputBuffer, int start
     float leftGain  = std::cos (panPos * juce::MathConstants<float>::halfPi);
     float rightGain = std::sin (panPos * juce::MathConstants<float>::halfPi);
 
+    // Update filter coefficients if cutoff changed
+    if (std::abs (filterCutoff - lastCutoff) > 1.0f || std::abs (filterReso - filterL.coefficients->coefficients[0]) > 0.01f)
+    {
+        float clampedCutoff = juce::jlimit (20.0f, (float) (sampleRate * 0.49), filterCutoff);
+        auto coeffs = juce::dsp::IIR::Coefficients<float>::makeLowPass (sampleRate, clampedCutoff, filterReso);
+        *filterL.coefficients = *coeffs;
+        *filterR.coefficients = *coeffs;
+        lastCutoff = filterCutoff;
+    }
+
+    bool useFilter = filterCutoff < 19900.0f; // Skip filter if fully open
+
     for (int i = 0; i < numSamples; ++i)
     {
         if (! playing) break;
@@ -375,6 +395,11 @@ void DrumPad::renderNextBlock (juce::AudioBuffer<float>& outputBuffer, int start
                 float s1 = sampleBuffer.getSample (srcCh, pos1);
                 float sampleVal = s0 + frac * (s1 - s0);
 
+                // Apply per-pad filter
+                if (useFilter)
+                    sampleVal = (ch == 0) ? filterL.processSample (sampleVal)
+                                          : filterR.processSample (sampleVal);
+
                 float channelGain = (ch == 0) ? leftGain : rightGain;
                 outputBuffer.addSample (ch, startSample + i, sampleVal * gain * channelGain);
             }