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/PluginEditor.cpp

@@ -211,7 +211,22 @@ void InstaDrumsEditor::timerCallback()
     for (auto* pc : padComponents)
         pc->repaint();
 
-    // Update VU meter from processor output levels
-    // (simplified: just repaint for now)
-    masterPanel.getVuMeter().repaint();
+    // Sync FX panel knobs -> processor atomic params
+    processor.compThreshold.store (fxPanel.getCompThreshold());
+    processor.compRatio.store     (fxPanel.getCompRatio());
+    processor.eqLo.store          (fxPanel.getEqLo());
+    processor.eqMid.store         (fxPanel.getEqMid());
+    processor.eqHi.store          (fxPanel.getEqHi());
+    processor.distDrive.store     (fxPanel.getDistDrive());
+    processor.distMix.store       (fxPanel.getDistMix());
+    processor.reverbSize.store    (fxPanel.getReverbSize());
+    processor.reverbDecay.store   (fxPanel.getReverbDecay());
+
+    // Sync master panel -> processor
+    processor.masterVolume.store (masterPanel.getMasterVolume());
+    processor.masterPan.store    (masterPanel.getMasterPan());
+    processor.masterTune.store   (masterPanel.getMasterTune());
+
+    // Update VU meter from processor
+    masterPanel.getVuMeter().setLevel (processor.vuLevelL.load(), processor.vuLevelR.load());
 }