Remove all LED pulsing - returning to stable baseline

Pulsing code was causing crashes (FastLED.show() RMT conflict with USB).
Removed:
- update() is now empty no-op (was calling mux_ptr->show() every 33ms)
- Removed set_bpm(), beat_interval_ms, bpm serial command
Kept:
- Per-pad LED colors with dim off state (working fine)
- All button/MIDI/expression pedal functionality

include/led_stub.h

@@ -33,7 +33,6 @@ private:
     static const uint8_t NUM_LEDS = 10;
     LedState led_states[NUM_LEDS];
     bool initialized;
-    uint32_t beat_interval_ms = 500;
     
 public:
     DefaultLedStub();
@@ -45,5 +44,4 @@ public:
     void update() override;
 
     void set_mux(PixelStompMux* mux);
-    void set_bpm(uint16_t bpm);
 };