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/led_stub.h

@@ -27,7 +27,7 @@ class PixelStompMux;
 
 class DefaultLedStub : public LedStub {
 private:
-    static const uint8_t NUM_LEDS = 10;
+    static const uint8_t NUM_LEDS = 8;
     LedState led_states[NUM_LEDS];
     bool initialized;