- 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
