docs(fm,srm): align federated terminate with SRM's partner rows and add partner_not_found

§L.3a and SRM §F.4a still described terminate as firing when the instance isn't in SRM's inventory; since ADR-0053's 2026-09-25 amendment SRM keeps a hosted_by: partner row, and that row is the trigger. §L.3a also had FM matching on external_resource_id, which holds the partner-issued appInstIdentifier; FM resolves the instance through its outbound install-app transaction instead. ADR-0053, FM RD §M.1 and SRM §F.4a gain completion_basis partner_not_found for a RemoveApp answered 404.