SubgraphDiscoveryProjection

Shared discovery channel on the mux. The transformer writes to it so the channel's lifetime stays tied to the mux (closed on mux.close() / failed on mux.fail()). The underscore prefix signals internal wiring: consumers should iterate subgraphs instead.

Async iterable of direct child stream handles of the root namespace. Wired onto GraphRunStream.subgraphs during root stream construction. For descendant namespaces, use filterSubgraphHandles to scope the same log.