Function load_etl_to_cursor 

fn load_etl_to_cursor(
    collector: &mut Collector<Vec<u8>, CompactU256>,
    total: usize,
    label: &str,
    append: impl FnMut(&[u8], U256) -> Result<(), DatabaseError>,
) -> Result<()>

Iterate a sorted ETL collector, deduplicate consecutive entries with the same key (keeping the last value), and call append for each unique entry.