Function backfill_one_bundle 

fn backfill_one_bundle(
    mstore: &Store,
    bundle_store: &Store,
    bundle_id: &str,
    skills_json: &str,
) -> usize

Carry one bundle’s inline skills across into the ref table.

Returns how many were bound. Never returns Err for bad data — see the module doc on why one bad bundle must not stop the rest.