Add PixelStomp MUX driver (DAT=9, LD=10, CLK=11, DI=12)

- New PixelStompMux class with shift register read/write
- Read 10 buttons via DI (active LOW)
- Drive 10 LEDs via DAT (shift out + latch)
- LED startup animation uses MUX
- Switch polling reads from MUX shift register
- MUX instance shared between switch and LED drivers

include/led_stub.h

@@ -23,6 +23,8 @@ struct LedState {
     bool active;
 };
 
+class PixelStompMux;
+
 class DefaultLedStub : public LedStub {
 private:
     static const uint8_t NUM_LEDS = 10;
@@ -34,4 +36,6 @@ public:
     void begin() override;
     void set_led_state(uint8_t note, uint8_t channel, uint8_t velocity) override;
     void clear_all() override;
+
+    void set_mux(PixelStompMux* mux);
 };