docs(troubleshooting): fix reserved 'default' name in buildx recovery

Troubleshooting.md

@@ -91,23 +91,23 @@ for m in $(mount | awk '$3 ~ /buildkit-mount/ {print $3}'); do
 done
 rm -rf ~/.docker/buildx/activity
 sudo systemctl start docker
-docker buildx create --use --name default
-docker buildx inspect --bootstrap
+docker buildx use default   # the bundled builder still exists — just point at it
 ```
 
 The `umount -l` step is the one most recipes online miss.
 
 **Fix — driver corruption (count == 0).**
 
-If `mount | grep -c buildkit-mount` already prints 0 and you still hit the wedge, the buildx driver state itself is inconsistent. Rebuild it:
+If `mount | grep -c buildkit-mount` already prints 0 and you still hit the wedge, the buildx driver state itself is inconsistent. Rebuild it under a non-reserved name:
 
 ```bash
-docker buildx rm default 2>/dev/null
-rm -rf ~/.docker/buildx/activity ~/.docker/buildx/instances/default
-docker buildx create --use --name default
-docker buildx inspect --bootstrap
+rm -rf ~/.docker/buildx/activity ~/.docker/buildx/instances/*
+docker buildx create --name decnet-builder --use --bootstrap
+docker buildx inspect
 ```
 
+`default` is reserved by Docker for the bundled builder — `buildx create --name default` errors out. Pick any other name.
+
 **How DECNET handles it.** The engine's `_compose_with_retry`:
 
 * Pre-flights leaked mounts before every build; if the count crosses 10, refuses to start and emits the leaked-mount recipe.