Multi-output routing (7 stereo buses) + VU meter fix

- 7 stereo output buses: Main, Kick, Snare, HiHat, Toms, Cymbals, Perc
- Each pad pre-assigned to appropriate bus (configurable via outputBus)
- Pads route to assigned bus if active, fallback to Main if not
- Master FX (limiter) applied to Main bus only
- isBusesLayoutSupported: Main must be stereo, aux can be stereo or disabled
- All buses enabled by default for REAPER multi-output detection
- VU meter: switched from RMS to peak measurement (getMagnitude)
- VU meter: sqrt scaling for better visibility on transient material
- VU meter: removed distracting dB scale markers
- VU meter: fast attack / medium release smoothing
- README updated with multi-output routing section

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

Source/DrumPad.h

@@ -50,6 +50,7 @@ public:
     float pitch = 0.0f;
     bool oneShot = true;
     int chokeGroup = -1;
+    int outputBus = 0;        // 0 = Main, 1 = Kick, 2 = Snare, etc.
     juce::Colour colour { 0xff00ff88 };
 
     // ADSR