feat(service-storage): the sys_attachment read gate writes one ReadableIds leaf per parent object and does no pre-scan; D5's three classes keep the probe (ADR-0139 E7, attachments)

feat(service-storage): the sys_attachment read gate writes one ReadableIds leaf per parent object and does no pre-scan; D5's three classes keep the probe (ADR-0139 E7, attachments) #15264