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/switch_stub.h

@@ -0,0 +1,36 @@
+#pragma once
+
+#include <cstdint>
+
+class SwitchStub {
+public:
+    virtual ~SwitchStub() {}
+    
+    virtual void begin() = 0;
+    virtual bool is_pressed(uint8_t switch_id) = 0;
+    virtual void configure_switch(uint8_t switch_id, uint8_t gpio_pin) = 0;
+    virtual void set_debounce_time(uint32_t time_ms) = 0;
+};
+
+struct SwitchState {
+    uint8_t id;
+    uint8_t gpio_pin;
+    bool current_state;
+    bool previous_state;
+    uint32_t last_change_time;
+    uint32_t debounce_time;
+};
+
+class DefaultSwitchStub : public SwitchStub {
+private:
+    static const uint8_t NUM_SWITCHES = 10;
+    SwitchState switch_states[NUM_SWITCHES];
+    bool initialized;
+    
+public:
+    DefaultSwitchStub();
+    void begin() override;
+    bool is_pressed(uint8_t switch_id) override;
+    void configure_switch(uint8_t switch_id, uint8_t gpio_pin) override;
+    void set_debounce_time(uint32_t time_ms) override;
+};