Function wire_downstream 

pub fn wire_downstream(
    upstream: &PluginRuntimeHandle,
    downstream_sender: NDArraySender,
)

Connect a downstream plugin’s sender to a plugin runtime’s output.

B13: the upstream’s array_output is the same Arc that every create_plugin_runtime* entry point registers in the WiringRegistry, so adding a sender here mutates the registry-tracked output — the registry remains the single source of truth for rewire_by_name.