Function find_remainder_leaves 

fn find_remainder_leaves(
    parquet_schema_desc: &SchemaDescriptor,
    root_idx: usize,
) -> Vec<usize>

Finds the Parquet leaves backing a JSON2 v2 remainder field.

The remainder is a sibling of explicitly materialized fields, so prefix matching a requested path cannot find it. These leaves are needed when an explicit path is absent or an explicitly materialized object may have additional children in the remainder.