Function persistence_for_data_path 

pub fn persistence_for_data_path(
    data_path: Option<&Path>,
) -> (Arc<dyn Persistence>, Option<Arc<LazyCypherSink>>)

Choose the appropriate persistence backend for a Uni instance.

Returns SystemLabelPersistence when data_path is a local filesystem path; falls back to uni_plugin_custom::NullPersistence when the instance is in-memory / object-store-backed (no local root for the JSON sidecar).

The optional second tuple element is the LazyCypherSink held by the SystemLabelPersistence (none for the null backend). The host’s Uni::build flow stashes it and calls LazyCypherSink::set_host_executor after the host is constructed, at which point subsequent declarations dual-write into the _DeclaredPlugin graph label.