Function compute_foreign_repo_overlap 

pub fn compute_foreign_repo_overlap(
    repo: &str,
) -> Result<Vec<ForeignRepoOverlapEntry>, String>

What: Foreign packages installed whose pkgname exists in repo’s sync DB.

Inputs:

• repo: Pacman repository name (same as pacman -Sl argument).

Output:

• Sorted overlap entries for stable UI.

Details:

• Read-only; does not mutate the system.

Errors

• Propagates failures from list_foreign_packages or [sync_repo_pkgnames_or_empty_if_repo_missing].