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/led_stub.cpp

@@ -1,7 +1,7 @@
 #include "led_stub.h"
 #include "pixel_stomp_mux.h"
-#include "midi_clock.h"
 #include <Arduino.h>
+#include <math.h>
 
 static PixelStompMux* mux_ptr = nullptr;
 
@@ -300,40 +300,46 @@ void DefaultLedStub::flash_activity() {
     // LED feedback is handled directly by set_led_state().
 }
 
+void DefaultLedStub::set_bpm(uint16_t bpm) {
+    if (bpm == 0) bpm = 120;
+    beat_interval_ms = 60000 / bpm;
+    Serial.printf("[LED] Pulse BPM set to %d (%dms)\n", bpm, beat_interval_ms);
+}
+
 void DefaultLedStub::update() {
     if (!initialized || !mux_ptr) return;
 
     uint32_t now = millis();
     static uint32_t last_pulse_update = 0;
 
-    if (now - last_pulse_update < 20) return;
+    if (now - last_pulse_update < 33) return;
     last_pulse_update = now;
 
-    bool clock_running = MidiClock::is_running();
-    float pulse = 1.0f;
-    if (clock_running) {
-        pulse = MidiClock::get_pulse();
-    }
+    float phase = (now % beat_interval_ms) / (float)beat_interval_ms;
+    float pulse = sinf(M_PI * phase);
 
+    bool active = false;
     for (int i = 0; i < NUM_LEDS; i++) {
         if (led_states[i].active) {
-            float brightness = clock_running ? (0.5f + 0.5f * pulse) : 1.0f;
+            active = true;
+            float brightness = 0.5f + 0.5f * pulse;
             uint32_t base = pad_base_colors[i];
             uint8_t r = (uint8_t)(((base >> 16) & 0xFF) * brightness);
             uint8_t g = (uint8_t)(((base >> 8) & 0xFF) * brightness);
             uint8_t b = (uint8_t)((base & 0xFF) * brightness);
             mux_ptr->set_led_color(i, r, g, b);
-        } else if (clock_running && i == 6) {
+        } else if (i == 6) {
+            active = true;
             float brightness = 0.3f + 0.7f * pulse;
             uint32_t base = pad_base_colors[6];
             uint8_t r = (uint8_t)(((base >> 16) & 0xFF) * brightness);
             uint8_t g = (uint8_t)(((base >> 8) & 0xFF) * brightness);
             uint8_t b = (uint8_t)((base & 0xFF) * brightness);
             mux_ptr->set_led_color(6, r, g, b);
-        } else {
-            apply_pad_color(i, 0);
         }
     }
 
-    mux_ptr->show();
+    if (active) {
+        mux_ptr->show();
+    }
 }