Replace MIDI clock with internal BPM timer for pixel 6 pulse

Revert:
- Remove midi_clock.h/cpp (was causing cross-core crash)
- Remove real-time MIDI message parsing from midi_transport.cpp

New approach:
- Pixel 6 (and all active LEDs) pulse to internal BPM timer
- Default 120 BPM (500ms beat interval)
- 'bpm <n>' serial command to change tempo (20-300)
- Only calls FastLED.show() when actively pulsing (avoids USB/WS2812 interference)
- 33ms update rate (30fps)
- Inactive LEDs left untouched (no unnecessary show() calls)

src/midi_transport.cpp

@@ -1,5 +1,4 @@
 #include "midi_transport.h"
-#include "midi_clock.h"
 #include <Arduino.h>
 #include "Adafruit_TinyUSB.h"
 
@@ -52,20 +51,6 @@ 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);