Add MUX diagnostic commands and GPIO probe

Serial commands:
- dump: show button and LED state
- ledon/ledoff: toggle all LEDs
- ledtest: sequential LED test
- read: raw button read
- probe: check pin states and test LD toggle

src/main.cpp

@@ -27,6 +27,52 @@ void midi_task(void* parameter) {
     }
 }
 
+void handle_serial_command(const String& cmd) {
+    if (cmd == "dump" || cmd == "d") {
+        mux.dump();
+    } else if (cmd == "ledon") {
+        for (int i = 0; i < 10; i++) {
+            mux.set_led(i, true);
+            delay(50);
+        }
+        Serial.println("[CMD] All LEDs ON");
+    } else if (cmd == "ledoff") {
+        mux.clear_all();
+    } else if (cmd == "ledtest") {
+        for (int i = 0; i < 10; i++) {
+            mux.set_led(i, true);
+            delay(200);
+            mux.set_led(i, false);
+            delay(100);
+        }
+        Serial.println("[CMD] LED test complete");
+    } else if (cmd == "read") {
+        uint16_t raw = mux.read_buttons();
+        Serial.printf("[CMD] Raw button state: 0x%03X (binary: ", raw);
+        for (int i = 9; i >= 0; i--) {
+            Serial.print((raw >> i) & 1);
+        }
+        Serial.println(")");
+    } else if (cmd == "probe") {
+        Serial.println("[CMD] Probing MUX pins...");
+        Serial.printf("  DAT (GPIO %d) = %d\n", 9, digitalRead(9));
+        Serial.printf("  LD  (GPIO %d) = %d\n", 10, digitalRead(10));
+        Serial.printf("  CLK (GPIO %d) = %d\n", 11, digitalRead(11));
+        Serial.printf("  DI  (GPIO %d) = %d\n", 12, digitalRead(12));
+
+        Serial.println("  Toggling LD pin...");
+        for (int i = 0; i < 5; i++) {
+            digitalWrite(10, HIGH);
+            delayMicroseconds(100);
+            digitalWrite(10, LOW);
+            delayMicroseconds(100);
+        }
+        Serial.printf("  DI after LD toggle: %d\n", digitalRead(12));
+    } else {
+        Serial.println("[CMD] Commands: dump, ledon, ledoff, ledtest, read, probe");
+    }
+}
+
 void setup() {
     Serial.begin(115200);
     delay(2000);
@@ -66,12 +112,20 @@ void setup() {
     
     Serial.println("=================================");
     Serial.println("  All systems ready");
-    Serial.println("  Waiting for USB MIDI host...");
+    Serial.println("  Type 'help' for diagnostics");
     Serial.println("=================================");
     Serial.flush();
 }
 
 void loop() {
     controller.update();
+
+    if (Serial.available()) {
+        String cmd = Serial.readStringUntil('\n');
+        cmd.trim();
+        cmd.toLowerCase();
+        handle_serial_command(cmd);
+    }
+
     delay(10);
 }