per-session presence is small and uncomplicated enough that a rollback
flag isn't load-bearing. backwards compat is already covered at the
protocol layer — older brokers reply unknown_message_type to
session_hello and the SessionBrokerClient marks itself closed for that
mesh, which is the same outcome the flag would have given. removing
the flag, the helper, and the conditional from the registry hook.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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