Set Subgroup Distribution Link

Link a whole subgroup to an employee distribution (NULL-scope members follow it, including items imported later), rescope it, or unlink it. Keys are stored verbatim — sentinels included — matching how assign_subgroup_to_activity addresses subgroups. Unlink zeroes the R&D of the members that were following the link, matching the per-item opt-out; updated_count is that many rows.