Rewrite MUX: WS2812C LEDs (NeoPixel) + 74HC165 buttons

PixelStomp MUX uses:
- WS2812C LEDs: one data line via DAT (GPIO 9), NeoPixel protocol
- 74HC165 shift register: LD/CLK/DI for reading 8 button states

Changes:
- Use Adafruit NeoPixel library for LED control
- Proper 74HC165 parallel-load shift-in for buttons
- 8 switches + 8 LEDs (was incorrectly 10)
- Diagnostic commands: dump, ledtest, red, green, blue, read

include/pixel_stomp_mux.h

@@ -1,22 +1,24 @@
 #pragma once
 
 #include <cstdint>
+#include <Adafruit_NeoPixel.h>
 
 class PixelStompMux {
 public:
-    static const uint8_t NUM_INPUTS = 10;
-    static const uint8_t NUM_OUTPUTS = 10;
+    static const uint8_t NUM_BUTTONS = 8;
+    static const uint8_t NUM_LEDS = 8;
 
     PixelStompMux(uint8_t dat_pin, uint8_t ld_pin, uint8_t clk_pin, uint8_t di_pin);
 
     void begin();
 
-    uint16_t read_buttons();
+    uint8_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 set_led_color(uint8_t index, uint8_t r, uint8_t g, uint8_t b);
+    void set_led_brightness(uint8_t brightness);
     void clear_all();
+    void show();
 
     void dump();
 
@@ -26,10 +28,9 @@ private:
     uint8_t pin_clk;
     uint8_t pin_di;
 
-    uint16_t last_button_state;
-    uint16_t current_led_state;
+    Adafruit_NeoPixel* strip;
 
-    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);
+    uint8_t last_button_state;
+
+    uint8_t shift_in_74hc165();
 };