Add MIDI clock-driven LED pulsing

- New MidiClock module: tracks 24 PPQN clock ticks, computes beat phase
  and smooth sine pulse from incoming MIDI Clock (0xF8)
- midi_transport: detect CIN=0xF real-time packets, route 0xF8 to
  MidiClock::tick(), 0xFA/FB to start(), 0xFC to stop()
- led_stub update(): every 20ms, modulates active LED brightness
  using pulse curve (50%-100%), pixel 6 always throbs when clock
  is running even if no loops active
- Aligned set_led_state() all paths to use pad_base_colors
- Removed dead activity flash code

include/led_stub.h

@@ -33,8 +33,6 @@ private:
     static const uint8_t NUM_LEDS = 10;
     LedState led_states[NUM_LEDS];
     bool initialized;
-    uint32_t activity_off_time = 0;
-    uint8_t saved_r = 0, saved_g = 0, saved_b = 0;
     
 public:
     DefaultLedStub();