fix: run john stress in parallel per GPU, fix chromium fullscreen, filter BMC virtual disks

- bee-john-gpu-stress: spawn one john process per OpenCL device in parallel
  so all GPUs are stressed simultaneously instead of only device 1
- bee-openbox-session: --start-fullscreen → --start-maximized to fix blank
  white page on first render in fbdev environment
- storage collector: skip Virtual HDisk* devices reported by BMC/iDRAC

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

iso/overlay/usr/local/bin/bee-openbox-session

@@ -24,7 +24,7 @@ chromium \
     --no-first-run \
     --disable-session-crashed-bubble \
     --disable-features=TranslateUI \
-    --start-fullscreen \
+    --start-maximized \
     http://localhost/ &
 
 exec openbox