A registered session behind its own async mutex. The registry map lock is
held only for lookup/insert/remove; the per-session mutex serializes all
processing (validate + storage append + commit) for that session ONLY —
RFC-MACP-0001 §8.1 requires acceptance serialization within a session,
never across sessions. Lock ordering: map lock BEFORE session mutex, and
never hold the map lock while awaiting a session mutex — snapshot the
Arcs, drop the map guard, then lock.