feat(credential): implement credential peer auth and trust propagation (#1968)

- add credential manager and RPC/CLI for generate/list/revoke
- support credential-based Noise authentication and revocation handling
- propagate trusted credential metadata through OSPF route sync
- classify direct peers by auth level in session maintenance
- normalize sender credential flag for legacy non-secure compatibility
- add unit/integration tests for credential join, relay and revocation

easytier/src/instance_manager.rs

@@ -423,6 +423,10 @@ fn handle_event(
                             instance_id
                         );
                     }
+
+                    GlobalCtxEvent::CredentialChanged => {
+                        event!(info, "[{}] credential changed", instance_id);
+                    }
                 }
             } else {
                 events = events.resubscribe();