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

include/led_stub.h

@@ -0,0 +1,37 @@
+#pragma once
+
+#include <cstdint>
+
+class LedStub {
+public:
+    virtual ~LedStub() {}
+    
+    virtual void begin() = 0;
+    virtual void set_led_state(uint8_t note, uint8_t channel, uint8_t velocity) = 0;
+    virtual void clear_all() = 0;
+    
+    virtual uint8_t note_to_index(uint8_t note) {
+        return note;
+    }
+};
+
+struct LedState {
+    uint8_t note;
+    uint8_t channel;
+    uint8_t velocity;
+    uint32_t timestamp;
+    bool active;
+};
+
+class DefaultLedStub : public LedStub {
+private:
+    static const uint8_t NUM_LEDS = 10;
+    LedState led_states[NUM_LEDS];
+    bool initialized;
+    
+public:
+    DefaultLedStub();
+    void begin() override;
+    void set_led_state(uint8_t note, uint8_t channel, uint8_t velocity) override;
+    void clear_all() override;
+};