Fix NeoPixel SEGV crash loop by checking /dev/mem permissions

The rpi_ws281x C library segfaults when it can't access /dev/mem,
which Python try/except can't catch. Guard with an os.access() check
so the service degrades gracefully (no LEDs) instead of crash-looping.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

modules/led/dw_led_controller.py

@@ -2,6 +2,7 @@
 Dune Weaver LED Controller - Embedded NeoPixel LED controller for Raspberry Pi
 Provides direct GPIO control of WS2812B LED strips with beautiful effects
 """
+import os
 import threading
 import time
 import logging
@@ -59,6 +60,14 @@ class DWLEDController:
         if self._initialized:
             return True
 
+        # Guard: the rpi_ws281x C library will SEGV if it can't access /dev/mem.
+        # Python try/except can't catch a segfault, so check permissions first.
+        if os.path.exists('/dev/mem') and not os.access('/dev/mem', os.R_OK | os.W_OK):
+            error_msg = "Cannot access /dev/mem — NeoPixel requires root permissions. Run the service with sudo."
+            self._init_error = error_msg
+            logger.warning(error_msg)
+            return False
+
         # Try standard NeoPixel library first (works on Pi 4 and earlier)
         # If that fails, fall back to Pi 5-specific library
         neopixel_module = None