fix(stress): stagger john GPU launches to prevent GWS tuning contention

When 8 john processes start simultaneously they race for GPU memory during
OpenCL GWS auto-tuning. Slower devices settle on a smaller work size (~594MiB
vs 762MiB) and run at 40% instead of 100% load. Add 3s sleep between launches
so each instance finishes memory allocation before the next one starts.

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

iso/overlay/usr/local/bin/bee-john-gpu-stress

@@ -191,7 +191,10 @@ CHOSEN_FORMAT=$(choose_format) || {
 
 echo "format=${CHOSEN_FORMAT}"
 PIDS=""
+_first=1
 for opencl_id in $(echo "${JOHN_DEVICES}" | tr ',' ' '); do
+    [ "${_first}" = "1" ] || sleep 3
+    _first=0
     ./john --test="${SECONDS}" --format="${CHOSEN_FORMAT}" --devices="${opencl_id}" &
     PIDS="${PIDS} $!"
 done