createSubgraphDiscoveryTransformer

Create the subgraph discovery transformer.

Registering this transformer against a mux replaces the legacy inline behavior that previously lived in StreamMux.push. The mux no longer knows about the subgraph factory: instead, this transformer is the single component that materializes stream handles and announces them on _discoveries.

Marked as a NativeStreamTransformer so the projection is treated as internal wiring (not merged into run.extensions and not auto-forwarded via StreamMux.wireChannels).