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/pixel_stomp_mux.h

@@ -0,0 +1,35 @@
+#pragma once
+
+#include <cstdint>
+
+class PixelStompMux {
+public:
+    static const uint8_t NUM_INPUTS = 10;
+    static const uint8_t NUM_OUTPUTS = 10;
+
+    PixelStompMux(uint8_t dat_pin, uint8_t ld_pin, uint8_t clk_pin, uint8_t di_pin);
+
+    void begin();
+
+    uint16_t read_buttons();
+    bool is_button_pressed(uint8_t index);
+
+    void write_leds(uint16_t led_state);
+    void set_led(uint8_t index, bool on);
+    void clear_all();
+
+    void dump();
+
+private:
+    uint8_t pin_dat;
+    uint8_t pin_ld;
+    uint8_t pin_clk;
+    uint8_t pin_di;
+
+    uint16_t last_button_state;
+    uint16_t current_led_state;
+
+    void shift_out(uint16_t data, uint8_t bits);
+    uint16_t shift_in(uint8_t bits);
+    void pulse_pin(uint8_t pin, uint8_t level, uint16_t duration_us = 1);
+};