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/main.cpp

@@ -89,6 +89,13 @@ void handle_serial_command(const String& cmd) {
     } else if (cmd == "exp") {
         Serial.printf("[CMD] EXP ADC=%d MIDI=%d\n",
                       exp_pedal.get_raw(), exp_pedal.get_value());
+    } else if (cmd.startsWith("bpm ")) {
+        uint16_t bpm = cmd.substring(4).toInt();
+        if (bpm >= 20 && bpm <= 300) {
+            led_driver.set_bpm(bpm);
+        } else {
+            Serial.println("[CMD] BPM must be 20-300");
+        }
     } else if (cmd == "usb") {
         Serial.printf("[CMD] USB mounted: %s\n", TinyUSBDevice.mounted() ? "YES" : "NO");
         Serial.printf("[CMD] USB ready: %s\n", TinyUSBDevice.ready() ? "YES" : "NO");
@@ -133,6 +140,7 @@ void handle_serial_command(const String& cmd) {
         Serial.println("  red/green/blue - solid colour");
         Serial.println("  pixel0/pixel1 - single pixel test");
         Serial.println("  exp        - expression pedal ADC/MIDI value");
+        Serial.println("  bpm <n>    - set LED pulse tempo (20-300, default 120)");
         Serial.println("  usb        - USB connection status and descriptor info");
         Serial.println("  gpiotest   - raw GPIO pin diagnostic");
         Serial.println("  rawled     - bit-bang WS2812 (no library)");
@@ -356,6 +364,9 @@ void setup() {
     Serial.println("[INIT] Initializing Expression Pedal...");
     exp_pedal.begin();
     
+    Serial.println("[INIT] Setting LED pulse tempo...");
+    led_driver.set_bpm(120);
+
     Serial.println("[INIT] Registering MIDI callbacks...");
     controller.begin();