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/switch_stub.h

@@ -21,6 +21,8 @@ struct SwitchState {
     uint32_t debounce_time;
 };
 
+class PixelStompMux;
+
 class DefaultSwitchStub : public SwitchStub {
 private:
     static const uint8_t NUM_SWITCHES = 10;
@@ -33,4 +35,6 @@ public:
     bool is_pressed(uint8_t switch_id) override;
     void configure_switch(uint8_t switch_id, uint8_t gpio_pin) override;
     void set_debounce_time(uint32_t time_ms) override;
+
+    void set_mux(PixelStompMux* mux);
 };