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/midi_transport.h

@@ -0,0 +1,45 @@
+#pragma once
+
+#include <cstdint>
+#include <functional>
+
+struct MidiEvent {
+    enum Type {
+        NOTE_ON,
+        NOTE_OFF,
+        CONTROL_CHANGE,
+        PROGRAM_CHANGE,
+        PITCH_BEND,
+        AFTERTOUCH_POLY,
+        AFTERTOUCH_CHAN,
+        SYSEX
+    } type;
+    
+    uint8_t channel;      // MIDI channel (1-16)
+    uint8_t data1;         // Note number or CC number
+    uint8_t data2;         // Velocity or CC value
+    uint32_t timestamp;   // Event timestamp
+};
+
+class UsbMidiTransport {
+public:
+    UsbMidiTransport();
+    ~UsbMidiTransport();
+    
+    bool begin();
+    void update();
+    
+    // Callback registration
+    void on_midi_receive(std::function<void(const MidiEvent&)> callback);
+    
+    // Send MIDI
+    void send_note_on(uint8_t channel, uint8_t note, uint8_t velocity);
+    void send_note_off(uint8_t channel, uint8_t note, uint8_t velocity);
+    void send_cc(uint8_t channel, uint8_t cc, uint8_t value);
+    
+private:
+    std::function<void(const MidiEvent&)> receive_callback;
+    bool initialized;
+    
+    void parse_midi_packet(const uint8_t* buffer, uint32_t size, MidiEvent& event);
+};