Function query_library_faceted_at 

pub fn query_library_faceted_at(
    storage_root: &Path,
    filter_json: &str,
    sort: &str,
) -> Result<Value, String>

Vault-free hypermedia library reads (storage path; no Sanctuary HostApi required). Faceted query + facet counts at storage_root (same JSON shape as HostApi).