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

src/midi_transport.cpp

@@ -1,4 +1,5 @@
 #include "midi_transport.h"
+#include "midi_clock.h"
 #include <Arduino.h>
 #include "Adafruit_TinyUSB.h"
 
@@ -51,6 +52,20 @@ void UsbMidiTransport::update() {
     while (usb_midi.available()) {
         uint8_t packet[4];
         if (usb_midi.readPacket(packet)) {
+            uint8_t cin = packet[0] & 0x0F;
+
+            if (cin == 0x0F) {
+                uint8_t rt = packet[1];
+                if (rt == 0xF8) {
+                    MidiClock::tick();
+                } else if (rt == 0xFA || rt == 0xFB) {
+                    MidiClock::start();
+                } else if (rt == 0xFC) {
+                    MidiClock::stop();
+                }
+                continue;
+            }
+
             MidiEvent event;
             parse_midi_packet(packet, 4, event);