The swarm-nats-auth role gains create+read on exactly the callout xkey's
two leaves, and no update, so a stored pair is never replaced.
swarm/public/ is readable by every token through bao's default policy.
swarm-bao-public-read-policy reads that policy, replaces only its own
marker-delimited block, and writes it back when it differs; it never
writes a policy it read back empty. Writing `default` is a new grant on
the granter's own policy, so on an existing store it lands only after
the bootstrap-token step re-runs; until then the unit says so and exits
0, and swarm-bao-granter-role restarts it on success.
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