Add raw GPIO test command to diagnose pin connectivity

src/main.cpp

@@ -80,6 +80,36 @@ void handle_serial_command(const String& cmd) {
         mux.set_led_color(1, 255, 255, 255);
         mux.show();
         Serial.println("[CMD] Pixel 1 WHITE (max brightness)");
+    } else if (cmd == "gpiotest") {
+        Serial.println("[CMD] === Raw GPIO Test ===");
+        uint8_t pins[] = {9, 10, 11, 12};
+        const char* names[] = {"DAT(9)", "LD(10)", "CLK(11)", "DI(12)"};
+        for (int p = 0; p < 4; p++) {
+            uint8_t pin = pins[p];
+            pinMode(pin, INPUT_PULLUP);
+            delay(10);
+            int hi = digitalRead(pin);
+            pinMode(pin, INPUT_PULLDOWN);
+            delay(10);
+            int lo = digitalRead(pin);
+            pinMode(pin, INPUT);
+            delay(10);
+            int fl = digitalRead(pin);
+            Serial.printf("  %s: PULLUP=%d PULLDOWN=%d FLOAT=%d\n", names[p], hi, lo, fl);
+        }
+        Serial.println("  Testing DAT(9) output toggle...");
+        pinMode(9, OUTPUT);
+        for (int i = 0; i < 5; i++) {
+            digitalWrite(9, HIGH);
+            delayMicroseconds(100);
+            int readback = digitalRead(9);
+            Serial.printf("    HIGH -> readback=%d\n", readback);
+            digitalWrite(9, LOW);
+            delayMicroseconds(100);
+            readback = digitalRead(9);
+            Serial.printf("    LOW  -> readback=%d\n", readback);
+        }
+        Serial.println("[CMD] === GPIO Test Complete ===");
     } else if (cmd == "help") {
         Serial.println("[CMD] Commands:");
         Serial.println("  dump    - show button states");
@@ -90,6 +120,7 @@ void handle_serial_command(const String& cmd) {
         Serial.println("  read    - raw button read");
         Serial.println("  red/green/blue - solid colour");
         Serial.println("  pixel0/pixel1 - single pixel test");
+        Serial.println("  gpiotest - raw GPIO pin diagnostic");
     } else {
         Serial.println("[CMD] Unknown command. Type 'help'");
     }