docs(troubleshooting): branch buildx recovery on leaked-mount count
prune+restart alone doesn't evict already-held mounts; full recipe
is stop-pkill-umount-start. Add a separate recipe for the count==0
case (driver state corruption, no mounts to clean) and call out the
strict signature DECNET uses to classify the wedge.
Spurious 'read-only file system' during docker compose up --build is
actually buildx leaking bind-mounts; document the symptom, the
mount-count sanity check, and the recovery commands. Companion to
the engine preflight that now catches this before the retry loop
burns 3 attempts on it.
