Streaming dispatch path for Subscription operations — counterpart to
invoke(). Same visibility + ACL checks (internal → handler_identity,
external → identity), then dispatches to the StreamingHandler. Pre-handler
errors (not-found, forbidden, INVALID_OPERATION_TYPE for non-Subscription
ops) yield a single error ResponseEnvelope via stream::once and end the
stream. Adds 6 unit tests covering dispatch, not-found, wrong-kind,
internal-from-external, ACL denied, and internal-call handler_identity ACL.
Refs ADR-049 §3, §5.
glm-5.2
acaa0513e4