Function build_baseline_deltas_json 

pub fn build_baseline_deltas_json<'a>(
    total_delta: i64,
    per_category: impl Iterator<Item = (&'a str, usize, usize, i64)>,
) -> Value

Build a JSON representation of baseline deltas for the combined JSON envelope.

Accepts a total delta and an iterator of per-category entries to avoid coupling the report module (compiled in both lib and bin) to the binary-only baseline module.