Switch from NeoPixel to FastLED for ESP32-S3 LED reliability

FastLED has better ESP32 support and handles timing/RMT properly.
- Use CRGB array instead of Adafruit_NeoPixel
- FastLED.show() is more reliable on ESP32-S3
- probe command now tests all LEDs at max brightness

include/pixel_stomp_mux.h

@@ -1,7 +1,7 @@
 #pragma once
 
 #include <cstdint>
-#include <Adafruit_NeoPixel.h>
+#include <FastLED.h>
 
 class PixelStompMux {
 public:
@@ -29,8 +29,7 @@ private:
     uint8_t pin_clk;
     uint8_t pin_di;
 
-    Adafruit_NeoPixel* strip;
-
+    CRGB leds[NUM_LEDS];
     uint8_t last_button_state;
 
     uint8_t shift_in_74hc165();