The tenant-salted column the range rides. Thread-keyed tables use their
thread_id (a minted ${tenantId}_${uuid}); row-keyed tables use their own
salted id. Any column whose values are salted ids works — that is the
whole reason the ids are salted at mint (docs/agent-memory-tenancy.md).
The PurgeTenantResult field this table's deleted-row count lands in.
Unprefixed table name; purgeTenant applies the host's tablePrefix.
One table purgeTenant reaps by the tenant-exact
[tid_, tid\x60)range predicate.