The substrate (L2PS subnet UID) already produces a unique value per
session in honest deployments, but the brief requires explicit rejection
of any session reusing a prior channelId. SDK ships an in-memory
default; production callers wire their own store (durable DB, chain
lookup, app-side persistence).
CH-6 per-session channelId uniqueness — pluggable persistence.
The substrate (L2PS subnet UID) already produces a unique value per session in honest deployments, but the brief requires explicit rejection of any session reusing a prior
channelId. SDK ships an in-memory default; production callers wire their own store (durable DB, chain lookup, app-side persistence).