Convert to PlatformIO/Arduino format

- Replace ESP-IDF structure with PlatformIO (platformio.ini)
- Move source files to src/ and include/
- Move libraries to lib/ directory
- Replace FreeRTOS with Arduino task/vTaskDelay
- Use Arduino MIDI library instead of raw TinyUSB
- Dual-core: MIDI on core 0, controller on core 1

lib/hal/led_stub.cpp

@@ -0,0 +1,45 @@
+#include "led_stub.h"
+#include <Arduino.h>
+
+DefaultLedStub::DefaultLedStub() : initialized(false) {
+    for (int i = 0; i < NUM_LEDS; i++) {
+        led_states[i].active = false;
+        led_states[i].velocity = 0;
+        led_states[i].note = 0;
+        led_states[i].channel = 0;
+        led_states[i].timestamp = 0;
+    }
+}
+
+void DefaultLedStub::begin() {
+    initialized = true;
+    Serial.println("[LED] Stub initialized (GPIO pins not configured yet)");
+}
+
+void DefaultLedStub::set_led_state(uint8_t note, uint8_t channel, uint8_t velocity) {
+    if (!initialized) return;
+    
+    uint8_t led_index = note_to_index(note);
+    
+    if (led_index < NUM_LEDS) {
+        led_states[led_index].note = note;
+        led_states[led_index].channel = channel;
+        led_states[led_index].velocity = velocity;
+        led_states[led_index].active = (velocity > 0);
+        led_states[led_index].timestamp = millis();
+        
+        Serial.printf("[LED] Note %d -> LED %d Ch %d Vel %d (%s)\n",
+                      note, led_index, channel, velocity,
+                      velocity > 0 ? "ON" : "OFF");
+    } else {
+        Serial.printf("[LED] Index out of range: %d (Note: %d)\n", led_index, note);
+    }
+}
+
+void DefaultLedStub::clear_all() {
+    for (int i = 0; i < NUM_LEDS; i++) {
+        led_states[i].active = false;
+        led_states[i].velocity = 0;
+    }
+    Serial.println("[LED] All LEDs cleared");
+}