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);
 };