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/switch_stub.cpp

@@ -0,0 +1,38 @@
+#include "switch_stub.h"
+#include <Arduino.h>
+
+DefaultSwitchStub::DefaultSwitchStub() : initialized(false) {
+    for (int i = 0; i < NUM_SWITCHES; i++) {
+        switch_states[i].id = i;
+        switch_states[i].gpio_pin = 0;
+        switch_states[i].current_state = false;
+        switch_states[i].previous_state = false;
+        switch_states[i].last_change_time = 0;
+        switch_states[i].debounce_time = 50;
+    }
+}
+
+void DefaultSwitchStub::begin() {
+    initialized = true;
+    Serial.println("[SW] Stub initialized (GPIO pins not configured yet)");
+}
+
+bool DefaultSwitchStub::is_pressed(uint8_t switch_id) {
+    if (!initialized || switch_id >= NUM_SWITCHES) {
+        return false;
+    }
+    return switch_states[switch_id].current_state;
+}
+
+void DefaultSwitchStub::configure_switch(uint8_t switch_id, uint8_t gpio_pin) {
+    if (switch_id >= NUM_SWITCHES) return;
+    switch_states[switch_id].gpio_pin = gpio_pin;
+    Serial.printf("[SW] Switch %d configured to GPIO %d\n", switch_id, gpio_pin);
+}
+
+void DefaultSwitchStub::set_debounce_time(uint32_t time_ms) {
+    for (int i = 0; i < NUM_SWITCHES; i++) {
+        switch_states[i].debounce_time = time_ms;
+    }
+    Serial.printf("[SW] Debounce time set to %lu ms\n", time_ms);
+}