Why: U3A succeeded (Backend e942a35b); U3B can now proceed using Backend CapabilityExplain DTO as the foundation. Evidence: U3A child card .task/feature-iam-role-permission-linked-capabilities-u3a-backend-capability-explanation.md (terminal_status=success, transport.commit=e942a35b). Impact: U3B dispatch ready; targets RDM projectFailure / permission explanation integration into create / member-add / conversation-work paths with 4 categorized errors. Refs: plan revision 3 U3; PR #628.