Function get_installation_diff_local 

pub fn get_installation_diff_local(
    conn: &impl DbQuery,
    old_membership: &GroupMembership,
    new_membership: &GroupMembership,
    membership_diff: &MembershipDiff<'_>,
) -> Result<InstallationDiff, IdentityDependencyError>

Compare exact verified snapshots without network or signature-verifier calls. A creation placeholder has an empty identity baseline. It never selects the receiver’s latest identity state. New membership references must be nonzero. A missing proof does not stop the scan, so Need names every missing proof.