Function collected_tuple_key 

pub fn collected_tuple_key(
    dims: &[String],
    ctx: &HashMap<String, Value>,
) -> String

A canonical, symmetric key for one upstream tuple over dims, derived from a per-tuple interpolation context ({dim_id: {alias: value}}). Both the storing side (the chained discovery, over its own for_each dims) and the reading side (a consuming for_each row, whose dims are a superset) compute the same key for the same tuple, so a collected list looks up correctly. Pure.