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.

include/led_stub.h

@@ -10,6 +10,8 @@ public:
     virtual void set_led_state(uint8_t note, uint8_t channel, uint8_t velocity) = 0;
     virtual void clear_all() = 0;
     virtual void set_led_brightness(uint8_t brightness) = 0;
+    virtual void flash_activity() {}
+    virtual void update() {}
     
     virtual uint8_t note_to_index(uint8_t note) {
         return note;
@@ -31,6 +33,8 @@ private:
     static const uint8_t NUM_LEDS = 10;
     LedState led_states[NUM_LEDS];
     bool initialized;
+    uint32_t activity_off_time = 0;
+    uint8_t saved_r = 0, saved_g = 0, saved_b = 0;
     
 public:
     DefaultLedStub();
@@ -38,6 +42,8 @@ public:
     void set_led_state(uint8_t note, uint8_t channel, uint8_t velocity) override;
     void clear_all() override;
     void set_led_brightness(uint8_t brightness) override;
+    void flash_activity() override;
+    void update() override;
 
     void set_mux(PixelStompMux* mux);
 };

src/app_task.cpp

@@ -46,6 +46,10 @@ void AppTask::process_midi_event(const MidiEvent& event) {
     Serial.printf("[APP] MIDI IN: Type=%d Ch=%d Data1=%d Data2=%d\n",
                   event.type, event.channel, event.data1, event.data2);
 
+    // Flash LED 0 white briefly on ANY MIDI input - visual activity indicator
+    // (visible without serial when connected to iPad)
+    led_driver->flash_activity();
+    
     uint8_t led_index = 0xFF;
     uint8_t midi_channel = event.channel;
     uint8_t midi_note = event.data1;

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;
+    }
+}

src/main.cpp

@@ -367,6 +367,7 @@ void setup() {
 }
 
 void loop() {
+    led_driver.update();
     controller.update();
 
     if (Serial.available()) {