Why: U1/U2/U7 done; U3 can start per dependency chain U1→U2→U7→U3→U4→U5→U6. U3 splits into U3A (Backend extension DTO + HTTP endpoint + error categorization) and U3B (RDM projectFailure integration). Evidence: U7 child card .task/feature-iam-role-permission-linked-capabilities-u7-canonical-two-trees.md (RDML1 d8cfc707, full RDM chain green). Impact: U3A dispatch ready; targets Backend explanation interface. Refs: plan revision 3 U3; PR #628.