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)

include/led_stub.h

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

include/midi_clock.h

@@ -1,19 +0,0 @@
-#pragma once
-#include <cstdint>
-
-class MidiClock {
-public:
-    static void tick();
-    static void start();
-    static void stop();
-    static float get_phase();
-    static float get_pulse();
-    static bool is_running();
-
-private:
-    static uint32_t last_tick_time;
-    static uint32_t tick_count;
-    static float tick_interval_ms;
-    static bool running;
-    static const uint8_t TICKS_PER_BEAT = 24;
-};