Add visual MIDI activity indicator

Flash LED 0 white for 30ms on any MIDI input.
Visible without serial - helps diagnose whether MIDI
is arriving when connected to iPad with Loopy Pro.

src/led_stub.cpp

@@ -258,3 +258,39 @@ void DefaultLedStub::set_led_brightness(uint8_t brightness) {
     mux_ptr->set_led_brightness(brightness);
     mux_ptr->show();
 }
+
+void DefaultLedStub::flash_activity() {
+    if (!initialized || !mux_ptr) return;
+
+    uint32_t now = millis();
+
+    // Save current LED 0 color if we just started flashing
+    if (now >= activity_off_time) {
+        saved_r = 0;
+        saved_g = 0;
+        saved_b = 0;
+        if (led_states[0].active) {
+            uint32_t color = launchpad_palette[led_states[0].velocity];
+            saved_r = (color >> 16) & 0xFF;
+            saved_g = (color >> 8) & 0xFF;
+            saved_b = color & 0xFF;
+        }
+    }
+
+    // Flash LED 0 white
+    mux_ptr->set_led_color(0, 255, 255, 255);
+    mux_ptr->show();
+    activity_off_time = now + 30;
+}
+
+void DefaultLedStub::update() {
+    if (!initialized || !mux_ptr) return;
+
+    // Turn off activity flash after 30ms
+    uint32_t now = millis();
+    if (activity_off_time > 0 && now >= activity_off_time) {
+        mux_ptr->set_led_color(0, saved_r, saved_g, saved_b);
+        mux_ptr->show();
+        activity_off_time = 0;
+    }
+}