Discovery owns an unmapped gateway whose environment is unreadable when the
exact-birth spawn ledger names its home, but the pause snapshot re-read only
the environment and wrote `home: null`. The readiness predicate read that as a
pre-home record and let another home's gateway with the same selectorless argv
retire the debt.
The snapshot, discovery and readiness now share one resolver
(update_fleet_scope.gateway_pid_home: environment, else ledger witness). A home
that no longer proves refuses the pause before any stop, and only an entry with
no `home` key (an older updater's token) keeps argv-only matching; a recorded
empty home is never matched.
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