Function merge_pack_tracked 

pub fn merge_pack_tracked(
    pack: &ContentPack,
    settings: &mut Settings,
) -> (MergeSummary, AddedContent)

Merge a validated pack into settings, recording exactly what was added. Append hints to each pool and routines to custom_routines, skipping ids that collide with a bundled starter or an already-present custom routine. Non-destructive — nothing is removed or overwritten. Returns the summary counts plus the concrete AddedContent for a later remove_content.